Prof. Dr. Koen Lenaerts 
As Bell notes, the evolution of the principle of equal treatment in the EU legal order has been governed by two different, albeit intertwined, dynamics. On the one hand, the principle of equal treatment, which originally operated as a federalising device, has progressively grown out of its internal market origins. Currently, that principle not only operates as a means of tearing down obstacles to the free movement of goods, persons and capital, but also implements the value of ‘equality’ on which the EU is founded. Indeed, for measures that fall within the scope of EU law, that principle prohibits discrimination based on sex, racial or ethnic origin, religion or belief, disability, age or sexual orientation (‘the widening of the principle of equal treatment’). On the other hand, the principle of equal treatment has become deeply rooted in the constitutional fabric of the EU. It is a general principle of EU law, which is enshrined in the Charter of Fundamental Rights of the European Union (the ‘Charter’). In addition to implementing one of the values in which the EU is founded, that principle is listed as one of the objectives that the EU must pursue. Moreover, that principle pervades the entire body of EU law and stands at the apex of the hierarchy of EU norms (‘the deepening of the principle of equal treatment’).
For the Court of Justice, the interpretation and application of the principle of equal treatment is of paramount constitutional importance. Indeed, the Court of Justice could not fulfil its mission of upholding the rule of law without honouring that principle, which is part and parcel of the very idea of true justice. That is why the Court of Justice strives to guarantee that, within the scope of EU law, comparable situations are not treated differently and that different situations are not treated in the same way unless such treatment is objectively justified (prohibition of discrimination).
By looking at some examples taken from the case law of the Court of Justice, the purpose of my contribution is thus to provide a general overview of the main features of that principle.
I. Test-Achats: the principle of equal treatment as a ground for review of secondary EU legislation.
Within the scope of application of EU law, the principle of equal treatment is binding not only on Member States and individuals, but also on the institutions, which may not adopt any criteria in breach of that principle. Thus, if secondary EU legislation runs counter to the principle of equal treatment, the Court of Justice would not hesitate to annul such legislation or declare it invalid.
In Test-Achats, the referring court asked the Court of Justice whether Article 5(2) of Directive 2004/113 was valid in light of the principle of equal treatment between men and women. Article 5(1) of Directive 2004/113 implements that principle in relation to ‘actuarial factors’. It provides that the differences in premiums and benefits arising from the use of sex as a factor in the calculation thereof must be abolished by 21 December 2007 at the latest. By way of derogation, Article 5(2) of Directive 2004/113 stated that it was permitted for Member States to introduce proportionate differences in individuals’ premiums and benefits where the use of sex was a determining factor in the assessment of risk based on relevant and accurate actuarial and statistical data. If a Member State made use of that option before 21 December 2007, its decision had to be reviewed five years after that date, account being taken of a Commission report. At the outset, the Court of Justice stressed that Directive 2004/113 expressly refers to Articles 21 and 23 of the Charter. Accordingly, the validity of Article 5(2) of Directive 2004/113 had to be determined in light of those two provisions. Next, the Court of Justice looked at the Treaty provisions which define the principle of equal treatment between men and women as a social objective to be attained by the EU, namely the second subparagraph of Article 3(3) TEU and Articles 8, 19(1) and 157(1) TFEU. In this regard, it pointed out that ‘[i]n the progressive achievement of that equality, it is the EU legislature which (…) determines when it will take action, having regard to the development of economic and social conditions within the [EU]’. This meant, for example, that, since the use of actuarial factors related to sex was a widespread practice in the Member States, it was permissible for the EU legislator to provide for the appropriate transitional period. This was actually the rationale underpinning Article 5(1) of Directive 2004/113 which sets 21 December 2007 as the deadline for the implementation of the principle of equal treatment between men and women in relation to actuarial factors. By contrast, Article 5(2) contained a derogation from that principle which was subject to no temporal limitation. ‘[G]iven that Directive 2004/113 is silent as to the length of time during which those differences may continue to be applied’, the Court of Justice observed, ‘Member States which have made use of the option are permitted to allow insurers to apply the unequal treatment without any temporal limitation’.
The Council supported the validity of Article 5(2) of Directive 2004/113, arguing that, in the context of certain branches of private insurance, the respective situations of male and female policyholders may not be regarded as comparable, given that, in light of statistical data, the levels of insured risk may be different for men and for women. However, the Court of Justice took a different view. According to Recitals 18 and 19 of Directive 2004/113, the latter favoured the application of rules of unisex premiums and benefits. Accordingly, ‘Directive 2004/113 is based on the premise that (…) the respective situations of men and women with regard to insurance premiums and benefits contracted by them are comparable’.
For the Court of Justice, Article 5(2) of Directive 2004/113, ‘which enables the Member States in question to maintain without temporal limitation an exemption from the rule of unisex premiums and benefits, works against the achievement of the objective of equal treatment between men and women, which is the purpose of Directive 2004/113, and is incompatible with Articles 21 and 23 of the Charter’. Consequently, that provision was held to be invalid upon the expiry of an appropriate transitional period, i.e. 21 December 2012.
II. Sturgeon: the principle of equal treatment and the principle of consistent interpretation
In accordance with a general principle of interpretation, ‘[an EU] act must be interpreted, as far as possible, in such a way as not to affect its validity and in conformity with primary law as a whole’. This means, for example, that all EU acts must be interpreted in accordance with the principle of equal treatment.
In this regard, in Sturgeon and Others, the Court of Justice was asked whether Regulation No. 261/2004 confers a right to compensation upon airline passengers in the event of delay. The wording of Regulation No. 261/2004 does not expressly create a right to compensation for those passengers whose flight is delayed, as opposed to passengers whose flight is cancelled, on whom such a right is explicitly conferred. Can this legislative silence be read as denying compensation to this category of passengers? The Court of Justice replied in the negative. It began by observing that, in the light of its objectives, Regulation No. 261/2004 does not exclude awarding compensation to passengers whose flight is merely delayed. Nor does Regulation No. 261/2004 rule out the possibility that, for the purposes of recognition of the right to compensation, both categories of passengers can be treated alike. Next, the Court of Justice noted that, in accordance with a general principle of interpretation, ‘[an EU] act must be interpreted in such a way as not to affect its validity’. This means that an EU act must be interpreted in compliance with superior rules of EU law, including the principle of equal treatment. Hence, where passengers whose flight is cancelled and passengers whose flight is delayed are in a comparable situation, Regulation No. 261/2004 must be interpreted in such a way as to treat both categories of passengers equally. To this effect, the Court of Justice noted that both categories of passengers suffer similar damage, consisting in a loss of time. In particular, the situation of passengers whose flight is delayed is comparable to that of passengers who are informed upon arrival at the airport that their flight is cancelled and subsequently re-routed in accordance with Article 5 of Regulation No. 261/2004. Since Article 5 (1) (c) (iii) of Regulation No. 261/2004 only provides for a right to compensation where the cancellation of a flight and its subsequent re-routing entail a loss of time equal to or in excess of three hours, the same should apply in the event of delay. Therefore, the Court of Justice ruled that in order for Regulation No. 261/2004 to comply with the principle of equal treatment, it had to be interpreted so as to grant a right to compensation to passengers whose flight is delayed and who reach their final destination three hours or more after the arrival time originally scheduled by the air carrier. Finally, the Court of Justice recalled that air carriers are not obliged to pay compensation where they manage to prove that cancellations and delays are caused by extraordinary circumstances.
III. Kücükdeveci: the horizontal direct effect of the principle of non-discrimination on grounds of age
The Court of Justice has ruled that some Treaty provisions may produce both vertical and horizontal direct effect. For example, this has been the case regarding the Treaty provisions on the free movement of workers. The same applies in relation to the principle of non-discrimination on grounds of sex in relation to pay.
More recently, in Kücükdeveci, the Court of Justice ruled that the principle of non-discrimination on grounds of age may also produce horizontal direct effect. In that case, the national court asked whether Paragraph 622 of the German Civil Code (BGB), which provides for the periods of notice on dismissal which employers are required to observe to be increased incrementally with the length of service, but which disregards periods of the employee’s employment before the age of 25, is compatible with EU law, notably Directive 2000/78 or the general principle of non-discrimination on grounds of age. If so, the national court also enquired whether this general principle may be applied to set aside Paragraph 622 BGB in a private dispute. The Court of Justice began by noting that the alleged discriminatory conduct took place after the period for transposition of Directive 2000/78 had expired. It also observed that Paragraph 622 BGB fell within the scope of application of that Directive given that the notice period is a condition of dismissal as provided for by Article 3 (1) (c) of Directive 2000/78. This meant that, since the contested legislation fell within the scope of application of that Directive, the latter had ‘the effect of bringing within the scope of [EU] law[Paragraph 622 BGB]’. Next, the Court of Justice found that, in accordance with Paragraph 622 BGB, employees who entered into an employment contract before the age of 25 benefited from a shorter period of notice than those who started working at a later age. Accordingly, the former category of employees was treated less favourably than the latter category just because of their age. The Court of Justice ruled that such a difference in treatment could not be justified as the contested provision failed to comply with the principle of proportionality, given that the extension of the notice period for dismissal according to the employee’s seniority in service was delayed for all employees who had joined the undertaking before the age of 25, even if the person concerned had a long length of service in the undertaking at the time of dismissal.
As there was a normative conflict between Paragraph 622 BGB and the principle of non-discrimination on grounds of age as given expression in Directive 2000/78, the Court of Justice was called upon to determine whether that provision could be set aside. After recalling that a Directive cannot produce horizontal direct effect, it observed that national law could not be interpreted ‘in the light of the wording and the purpose of the directive in question’, since that option had been ruled out by the national court. Next, referring to its previous ruling in Mangold, the Court of Justice held that ‘Directive 2000/78 merely gives expression to, but does not lay down, the principle of equal treatment in employment and occupation, and that the principle of non-discrimination on grounds of age is a general principle of [EU] law in that it constitutes a specific application of the general principle of equal treatment’. In the key passage of the judgment, which merits full quotation, the Court of Justice ruled that:
‘ In those circumstances, it for the national court, hearing a dispute involving the principle of non-discrimination on grounds of age as given expression in Directive 2000/78, to provide, within the limits of its jurisdiction, the legal protection which individuals derive from [EU] law and to ensure the full effectiveness of that law, disapplying if need be any provision of national legislation contrary to that principle’.
Before the Court of Justice delivered its judgment in Kücükdeveci, several objections had been raised against the horizontal application of general principles of EU law. For instance, AG Ruiz-Jarabo Colomer, whose second Opinion in Pfeiffer favoured the horizontal direct effect of Directives with a view to excluding conflicting national rules, subsequently argued in Michaeler that the horizontal application of general principles of EU law would ‘convert (…) typical [EU] acts into merely decorative rules which may be easily replaced by the general principles’. Likewise, AG Kokott opined in Kofoed that parties should not be able to rely on a general principle of EU law where the latter is given specific effect and expressed in a concrete manner in a Directive. Since the content of general principles is ‘much less clear and precise’, the Advocate General considered that, for situations falling within the scope of a Directive, ‘there would [otherwise] be a danger (…) that the harmonisation objective of [the] Directive (…) would be undermined and the legal certainty (…) which it seeks to achieve would be jeopardised’. 
In the context of Directive 2000/78, AG Mazák in Palacios de la Villa observed that if general principles of EU law could be invoked independently of EU implementing legislation, not only would legal certainty be threatened –an obligation not laid down in national law being imposed on certain subjects of the law– but the vertical and horizontal allocation of powers laid down in Article 19 TFEU would also be disturbed. The vertical allocation of powers would be threatened because the unanimity procedure laid down in that provision protects the competences of the Member States. The horizontal allocation of powers would suffer because the application of general principles of EU law between private parties runs the risk of rendering meaningless the choice of the Council to enact EU legislation lacking horizontal direct effect. More recently, in Dominguez, AG Trstenjak criticised the approach followed by the Court of Justice in Kücükdeveci. In this regard, she argued that such approach could give rise ‘to irreversible “ossification” of [the] legislative content [set out in Directive 2000/78]. As a result of incorporating more and more legislative content from a directive within the scope of protection of a general principle, the legislature would be deprived of the ability to make amendments to the directive, especially as such legislative content would then be elevated to the status of primary law, upon which it cannot impinge’.
In Kücükdeveci, AG Bot argued that the horizontal application of the general principle of non-discrimination on grounds of age did not encroach upon the powers of the EU legislator. He posited that the normative yardstick for evaluating whether national law complies with EU law remains the relevant Directive and not the general principle enshrined therein. It is only at a later stage that the general principle might become relevant, that is, when assessing the implications that flow from the fact that a national provision is in breach of the Directive. In examining whether a national provision is discriminatory, the Court of Justice will confine itself to interpreting Directive 2000/78. In particular, it will take due account of the limitations or derogations to the principle of equal treatment introduced by the EU legislator. Thus, the Advocate General suggests that this two-step analysis does not impinge upon the prerogatives of the EU legislator. Prechal seems to agree with AG Bot. She states that a Directive implementing a general principle should facilitate, rather than limit, ‘the application and implementation of the general principle’ contained therein. Accordingly, as a sanction imposed on the infringing Member State, national provisions conflicting with the Directive should be set aside. However, in order not to upset the vertical and horizontal allocation of powers provided for by the Treaty, she argues that not only should the general principle be ‘operational’– i.e. contain judicially manageable standards –, but the Court of Justice should also follow ‘very closely the letter of the Directive, (…) remain[ing] nearly entirely within the scope of what the legislator provided for’. This is precisely what the Court of Justice did in Mangold and Kücükdeveci. First, the principle of equality is ‘often as such operational’. Second, the Court of Justice remained close to the terms of the Directive.
In addition, as I have argued elsewhere, these objections seem to overlook the fact that general principles of EU law enjoy a ‘constitutional status’. In light of the hierarchy of norms, this means that whether a general principle produces horizontal direct effect is a determination that takes place at the level of ‘primary law’. Such a determination does not fall within the purview of the EU legislator, nor of the Member States. It would take a Treaty amendment for the political process to interfere with it. Defrenne and Angonese illustrate this point.
In the former case, the Court of Justice ruled that the general principle of equal pay for equal work – grounded in ex Article 119 EEC (now Article 157 TFEU) – may produce horizontal direct effect. After looking at the aim, the nature and the place of the principle of equal pay for equal work in the scheme of the Treaty, the Court of Justice held that this principle is ‘mandatory in nature’ and accordingly, applies to public authorities and private individuals alike. By contrast, not only was Directive 75/117, which sought to improve the legal protection of workers suffering from unequal pay caused by sex discrimination, irrelevant to determine whether the principle of equal pay for equal work could produce horizontal direct effect, but the Court of Justice actually pointed out that in no way could Directive 75/117 reduce the effectiveness and the temporal scope of that principle. Likewise, in Angonese the Court of Justice observed that Regulation No. 1612/68 which implemented the principle of free movement of workers as laid down in ex Article 39 EC (now Article 45 TFEU) was not applicable to a competition for a post organised by a private bank. This circumstance, however, did not prevent the principle of free movement of workers – a specific application of the general principle of non-discrimination on grounds of nationality – from producing horizontal direct effect. The Court of Justice reasoned that its findings in Defrenne could, a fortiori, apply to the free movement of workers, since both principles are ‘mandatory in nature’ and seek to combat discrimination, albeit based on different grounds, on the labour market. In the field of social law, could Defrenne support the horizontal application of other ‘constitutional categories’ of the principle of non-discrimination which are ‘mandatory in nature’ but are not laid down in a Treaty provision? In this regard, one could argue that in order to preserve the vertical and horizontal allocation of powers, only those general principles of EU law which are enshrined in a Treaty provision may produce horizontal direct effect. Stated differently, in the absence of a Treaty provision, Defrenne may not be relied upon. Yet, this argument does not seem convincing. A close reading of Defrenne reveals that the wording of ex Article 119 EEC did not play a major role in the rationale of the Court of Justice.  Indeed, the Court of Justice did not focus on whether this Treaty provision was sufficiently precise to produce direct effect, preferring, instead, to ‘identify and isolate’ the general principle of equal pay for equal work. Most importantly, Defrenne shows that in deciding the horizontal application of the principle of equal pay for equal work, the Court of Justice was respectful of the prerogatives of the EU legislator and of the Member States. To this effect, the Court of Justice drew a distinction between situations where a ‘purely legal analysis’ sufficed to detect the presence of sex discrimination and complex situations where such a presence could not be ascertained unless legislative measures were adopted. While in relation to the former type of situations, the Court of Justice is in a position to hold that the general principle of equal pay for equal work produces horizontal direct effect, in the latter type of situations the Court of Justice is not. Therefore, in so far as this distinction is complied with, the vertical and horizontal allocation of powers is not disturbed by the horizontal application of a general principle. This is so regardless of whether the general principle is grounded in a Treaty provision.
It is in this sense that the Court of Justice confirmed in Kücükdeveci that the general principle of non-discrimination on grounds of age, now enshrined in Article 21 of the Charter, is to be applied horizontally in an employment relationship covered by Directive 2000/78.
IV. Bressol: the principle of equal treatment and EU citizenship
Every citizen of the Union has the right not to suffer discrimination on grounds of nationality in all situations falling ‘within the scope of application of the Treaties’. Such situations include those involving the exercise of the fundamental freedoms guaranteed by the Treaties, as well as those involving the exercise of the right enjoyed by citizens of the Union to move and reside within the territory of the Member States (see Article 21 TFEU [ex Article 18 EC]). Accordingly, a person can rely on the provisions of the Treaties if he or she, being a national of a Member State and thus a citizen of the Union, lawfully resides in the territory of another Member State.Moreover, Article 18 TFEU also applies where a family member of a citizen of the Union has made use of his or her right of free movement (as opposed to the citizen himself or herself), where this results in unequal treatment.
The exercise of the fundamental right to free movement enables citizens of the Union to claim equal treatment in a broad range of areas. For example, the Court of Justice applies Article 18 TFEU to the conditions governing access to education. Consequently, it is, in principle, contrary to the principle of equal treatment for a Member State to provide free access to higher education for resident students, whilst applying a ‘numerus clausus’ to students coming from other Member States. The ruling of the Court of Justice in Bressol illustrates that point.
In that case, the Court of Justice was asked by the Belgian Constitutional Court to examine the compatibility of the decree of the French Community (‘the 2006 decree’) – which regulated the number of students in certain programmes in the first two years of undergraduate studies in higher education – with the Treaty provisions on EU citizenship. The system of higher education of the French Community is based on free access to education, i.e. there are no entry exams. However, in recent years, French students, who had failed to pass the entry exams in France, have gone to study at the Universities of the French Community. The number of students having become too large, in particular in medical and paramedical courses, the authorities of the French Community thought that, having regard to the budgetary, human and material resources available to the teaching institutions concerned, such an influx of students was jeopardising the quality of teaching – and, because of the nature of the programmes at issue, public health. Accordingly, in relation to nine medical or paramedical programmes, the French Community adopted the 2006 decree which provided a numerus clausus for enrolment by non-resident students who were selected by the drawing of lots, whilst resident students continued to enjoy free access to the courses referred thereto. In order to qualify as a resident student, a double condition had to be fulfilled. ‘Essentially, “residents” [were] persons who both [had] their principal residence in Belgium and [had] a right of permanent residence in Belgium’. Moreover, the number of non-resident students in those courses could not exceed a 30% threshold. Mr Bressol and other students, most of them French nationals, brought an action before the Belgian Constitutional Court contesting the constitutionality of the 2006 decree. They posited that that decree violated the principle of equality by treating resident and non-resident students differently, for no valid reason.
At the outset, the Court of Justice stressed that EU law does not detract from the Member States the power to organise their education systems and vocational training. A Member State is free to opt for a system based on free access or for a system which lays down a numerus clausus of students. However, in so doing, it must comply with EU law, in particular with the Treaty provisions on EU citizenship. Next, the Court of Justice found that the 2006 decree put non-resident students at a disadvantage vis-à-vis resident students, since only the latter continued to enjoy free access to any of the nine medical or paramedical courses referred to. Since the condition of residence was more easily met by Belgians than by students of other nationalities, the 2006 decree created a difference in treatment indirectly based on nationality that needed to be justified.
As to the justification of the 2006 decree, the Belgian Government argued that it sought to counter the excessive burdens on the financing of higher education brought about by non-resident students. However, observing that the financing of higher education did not depend on the total number of students but was based on a system of a ‘closed envelope’, the Court of Justice dismissed that justification.
In addition, the Belgian Government posited that the 2006 decree aimed to ensure the quality and continuing provision of medical and paramedical care within the French Community. It argued that the large numbers of non-resident students were likely ultimately to reduce the quality of teaching in medical and paramedical courses which require a significant amount of hours of practical training. In the same way, those large numbers may also bring about a shortage of qualified medical personnel throughout the territory which would undermine the system of public health within the FrenchCommunity, given that after completing their studies, non-resident graduates tend to return to their country of origin to exercise their profession there, whilst the number of resident graduates remains too low in some specialities. The Court of Justice recognised as legitimate the public health concerns raised by the Belgian Government. However, it provided a detailed framework of analysis that the referring court had to follow in order to determine whether there were genuine risks to the protection of public health and thus, whether the 2006 decree complied with EU law.
First, in assessing those risks, the referring court had to take into consideration ‘the fact that the link between the training of future health professionals and the objective of maintaining a balanced high-quality medical service open to all is only indirect and the causal relationship less well established than in the case of the link between the objective of public health and the activity of health professionals who are already present on the market’. Second, the Court of Justice pointed out that, whilst the Member State concerned does not have to wait for the risks to the protection of public health to materialise, it must, however, show that those risks actually exist. Third, the Court of Justice held that, for each of the nine courses covered by the 2006 decree, the analysis undertaken by the referring court had to determine the maximum number of students who can be trained at a level which complies with the desired training quality standards, as well as the number of graduates which is necessary to ensure adequate public health services. In determining those numbers, the analysis may not focus on one or the other group of students but it must take into account the number of non-resident students who decide to practice in Belgium, the number of resident students who decide to work in a Member State other than Belgium, and the number of healthcare service providers who may come to work in Belgium.
As to the proportionality of the contested measure, the Court of Justice held that the referring court had to verify whether a system of numerus clausus for non-resident students can really bring about an increase in the number of graduates ready to ensure the future availability of public health services within the French Community. As to the necessity of the measure, the Court of Justice ruled that it was for the referring court to ascertain whether there were less restrictive means of encouraging students who study in the French Community to establish themselves there. Finally, the Court of Justice compelled the referring court to verify whether the system of selection for non-resident students (based on chance rather than on merits) was necessary to attain the objectives pursued.
V. Chatzi, Audiolux and NCC Construction Danmark: Constitutionally prohibited grounds of discrimination and the relationship with the EU legislator
In defining and implementing its policies and activities, the EU shall aim to combat discrimination based on sex, racial or ethnic origin, religion or belief, disability, age or sexual orientation (Article 10 TFEU). Under Article 19 TFEU [ex Article 13 EC], introduced by the Amsterdam Treaty), the Council, acting unanimously in accordance with a special legislative procedure and after obtaining the consent of the European Parliament, is to take appropriate action within the limits of the powers conferred upon the EU by the Treaties to combat discrimination based on these grounds. Accordingly, the Council has adopted directives prohibiting discrimination on the basis of racial or ethnic origin and a general framework for equal treatment in employment and occupation. The purpose of the second directive is to combat any form of discrimination on the grounds of religion or belief, disability, age or sexual orientation as regards employment and occupation.
The Treaty of Nice made it possible for the European Parliament and the Council to adopt, under the ordinary legislative procedure, incentive measures, excluding any harmonisation of the laws and regulations of the Member States, to support action taken by the Member States (Article 19(2) TFEU).Where a person suffers discrimination outside that context on the basis of the specified (or other) criteria, he or she can bring an action on the basis of the general principle of equal treatment, unless, as far as EU law is concerned, his or her situation contains no link with EU law.
For example, in Chatzi, the referring court asked whether clause 2.1 of the framework agreement on parental leave could be interpreted as meaning that the birth of twins confers entitlement to a number of periods of parental leave equal to the number of children born or whether it must be interpreted as meaning that their birth confers entitlement, like the birth of a single child, to just a single period of parental leave. After looking at the wording of clause 2.1 and after interpreting it literally, teleologically and contextually, the Court of Justice held that that provision does not require that entitlement to a number of periods of parental leave equal to the number of children born be automatically recognised in the event of the birth of twins. Next, the Court of Justice held that Clause 2.1 had to be interpreted in light of the principle of equal treatment. In that regard, it noted that it was difficult to establish the group of persons with whom the parents of twins may be compared. However, ‘whilst it cannot be denied that the task of bringing up twins entails greater effort and therefore is not comparable to care of a single child, it also cannot be ignored that the fact that twins grow up and develop in parallel entails synergies and that, consequently, the task of bringing them up is not necessarily comparable to the task that bringing up two children of different ages involves’. That being said, the Court of Justice held that, ‘read in the light of the principle of equal treatment, this clause obliges the national legislature to establish a parental leave regime which, according to the situation obtaining in the Member State concerned, ensures that the parents of twins receive treatment that takes due account of their particular needs’.
Moreover, it follows from Audioluxand NCC Construction Danmarkthat it falls to the EU legislature (or, where appropriate, to the national legislature) to identify the criteria on which differentiation between individual cases cannot legitimately be based. Yet, as Mangold demonstrates, the general principle of equality may be relied upon without further legislative intervention in relation to constitutionally prohibited forms of discrimination (e.g. discrimination on grounds of nationality, sex, age). The distinction between constitutionally prohibited forms of discrimination and other sets of circumstances calling for legislative intervention is a positive development. It shows that, beyond constitutionally prohibited forms of discrimination, the general principle of equal treatment cannot be relied upon to replace legislative choices. Otherwise, the Court of Justice would risk being dragged into policy-making based on its own conception of redistributive justice. However, this does not mean that the general principle of equality is confined to protecting constitutional situations. As NCC Construction Danmark shows, it only means that, when legislative discretion is involved, that principle intervenes at a later stage: as an ex post review of the internal consistency of the choices made by the legislature.
- Kamberaj : the principle of equal treatment and third country nationals
The Court of Justice has made clear that Article 18 TFEU does not prohibit differences in treatment between nationals of Member States and nationals of third countries. In Vatsouras, the Court of Justice ruled that ‘[Article 18 TFEU] concerns situations coming within the scope of [EU] law in which a national of one Member State suffers discriminatory treatment in relation to nationals of another Member State solely on the basis of his nationality and is not intended to apply to cases of a possible difference in treatment between nationals of Member States and nationals of non-member countries’.
However, the EU legislator has passed secondary legislation which seeks to guarantee that Member States do not discriminate against third-country nationals who are long-term residents. Notably, Article 7 of Directive 2003/109 lists the areas in which such treatment must be granted to long-term residents. Nevertheless, it also lists the situations in relation to which Member States may place restrictions on that principle. In Kamberaj, the Court of Justice was called upon to interpret the principle of equal treatment as given expression in that Directive.
In that case, the referring court asked whether a Member State could discriminate against third-country nationals who are long-term residents in relation to housing benefits (for the case at hand, it was a contribution by national authorities to the payment of the rent for low income tenants). It is true that Article 7(1)(d) of Directive 2003/109 provides that long-term residents shall enjoy equal treatment with nationals as regards ‘social security, social assistance and social protection’. However, that provision states that those concepts are ‘defined by national law’. For the Court of Justice, this meant that, since the EU legislature wished to respect the differences between the Member States, the concepts of ‘social security’, ‘social protection’ and ‘social assistance’ are defined by national law, subject however to compliance with EU law. It follows that it was for the national court to assess whether the housing benefit in question fell within the fields covered by Directive 2003/109, taking into account both the integration objective pursued by that Directive and the provisions of the Charter. In addition, Article 7(4) of Directive 2003/109 states that ‘Member States may limit equal treatment in respect of social assistance and social protection to core benefits’. Logically, the question was whether, in the event of the Member State concerned considering that housing benefits are part of its social assistance, those benefits could qualify as ‘core benefits’. In that regard, the Court of Justice held that, in so far as Directive 2003/109 does not contain an exhaustive list of core benefits, it could not be excluded that the housing benefit in question fell within that concept, to which the principle of equal treatment had to be applied. Moreover, when defining the benefits that belong to the core of its social assistance, the Member State concerned must take into account the objective pursued by Directive 2003/109, namely the integration of third-country nationals who have resided legally and continuously in the Member States. The concept of core benefits must also be interpreted in conformity with the principles of the Charter, which recognises and respects the right to social and housing assistance so as to ensure a decent existence for all those who lack sufficient resources.
VII. Evolution and future challenges: same-sex marriages and bioethics
So far, the EU Courts have deferred to the Member States and, as the case may be, to the EU legislature the question whether married couples and same-sex partnerships should be treated alike.The Courts referred to the legislator’s power to make societal choices connected with the assessment of such discrimination. The Court of Justice did not consider that the situation of a married official was comparable to the same-sex partnerships recognised by some Member States. However, the case law may evolve further. According to the most recent Staff Regulations, EU officials in a non-marital relationship recognised by a Member State as a stable partnership who do not have legal access to marriage should be granted the same range of benefits as married couples. For some benefits, this also extends to other kinds of non-marital partnerships recognised by a Member State. Likewise, recent EU legislation on the right of citizens of the Union and their family members to move and reside freely within the territory of the Member States includes among the “family member” of a citizen the registered partner if the legislation of the host Member State treats registered partnership as equivalent to marriage. It is also contrary to the principle of equal treatment for the EU institutions to make the grant of an allowance for an official’s children who have lost their other parent dependent upon the condition that the official was married to that other parent.
More recently, the Court of Justice had held that, in so far as national law treats marriage and same-sex partnerships alike, any discriminatory treatment regarding benefits deriving from an employment relationship would be contrary to the principle of non-discrimination on grounds of sexual orientation as given expression in Directive 2000/78. For example, if under national law marriage and same-sex partnerships stand on an equal footing, a national measure limiting survivors’ benefits under a compulsory occupational pensions scheme to surviving spouses would run counter to the principle of equal treatment.
Moreover, the application of the principle of equal treatment in matters relating to bioethics may constitute a challenging task for the Court of Justice in the near future. For example, in pending case C.D., the Court of Justice will be called upon to decide whether the Pregnant Workers Directive 92/85/EEC provides a right to receive maternity leave to an intended mother who has a baby through a surrogacy arrangement, or whether such a right derives from the principle of non-discrimination on grounds of sex as given expression in the Recast Equal Treatment Directive 2006/54/EC. Similarly, in pending case Z v A, the referring court asks whether Directive 2006/54/EC may be interpreted as meaning that there is discrimination on the ground of sex where a woman – whose genetic child has been born through a surrogacy arrangement, and who is responsible for the care of her genetic child from birth – is refused paid leave from employment equivalent to maternity leave and/or adoptive leave. It also asks whether Directive 2000/78/EC may be interpreted as meaning that there is discrimination on the ground of disability where a woman – who suffers from a disability which prevents her from giving birth, whose genetic child has been born through a surrogacy arrangement, and who is responsible for the care of her genetic child from birth – is refused paid leave from employment equivalent to maternity leave and/or adoptive leave.
VIII. Concluding remarks
In summary, when the Court of Justice is called upon to interpret and, where appropriate, to apply the principle of equal treatment, it embarks on a challenging task. First, the Court of Justice must establish the appropriate comparator. This is not always an obvious thing to do. As Chatzi, C.D. and Z v A reveal, it may be difficult to establish the group of persons with whom the parents of twins or an intended mother may be compared.
Second, once it has been established that comparable situations are treated differently (or that the same treatment applies to different situations), the Court of Justice must examine whether such a difference in treatment is objectively justified. Where constitutionally prohibited forms of discrimination are involved, no further legislative intervention is required for the Court of Justice to examine the compatibility of such a difference in treatment with EU law. By contrast, where a difference in treatment does not involve those forms but is the result of legislative choices, the ECJ must examine the internal consistency of those choices. It is thus by looking at the rationality of the choices made by the EU or, as the case may be, the national legislator that the Court of Justice will established whether the principle of equal treatment has been infringed.
Third, compliance with the principle of equal treatment requires compliance with the principle of proportionality. As Bressol illustrates, this may entail a complex examination of the facts of the case at hand. In the context of the preliminary reference procedure, this means that the role of the Court of Justice is limited to providing a framework of analysis that the national court must apply.
Finally, EU law and national law falling within the scope of that law must be interpreted in light of the principle of equal treatment. Where such interpretation is not possible, the contested EU measure will be annulled or, as the case may be, declared invalid. However, in a private dispute, the setting aside of national law conflicting with a directive may only take place where the principle of equal treatment is by itself sufficiently operational and mandatory in nature, i.e. where it gives concrete expression to one of the constitutionally prohibited forms of discrimination.
Към резюмето на български език: ПРИНЦИПЪТ НА РАВНОТО ТРЕТИРАНЕ И СЪДЪТ НА ЕВРОПЕЙСКИЯ СЪЮЗ
 Vice-President of the Court of Justice of the European Union, and Professor of European Union Law, Leuven University. All opinions expressed herein are strictly personal to the author.
 M. Bell, ‘The Principle of Equal Treatment: Widening and Deepening’ in P. Craig and G. de Búrca (eds), The Evolution of EU law, 2nd Ed. (Oxford, OUP, 2011) 611 et seq.
 Article 2 TEU reads as follows:
‘The Union is founded on the values of respect for human dignity, freedom, democracy, equality, the rule of law and respect for human rights, including the rights of persons belonging to minorities.
These values are common to the Member States in a society in which pluralism, non-discrimination, tolerance, justice, solidarity and equality between women and men prevail.’
  OJ C 326/02. Chapter III of the Charter, which deals with equality before the law (Article 20), non-discrimination (Article 21), cultural, religious and linguistic diversity (Article 22), equality between men and women (Article 23), the rights of the child (Article 24), the rights of the elderly (Article 25) and integration of persons with disabilities (Article 26).
 See, regarding the promotion of equality between men and women, Articles 3(3) TEU and 157 TFEU.
 See, generally, K. Lenaerts and P. Van Nuffel, European Union Law, 3rd Ed (London, Sweet & Maxwell, 2011), Chapter 7, 156 et seq.
 Directive 2004/113 implements the principle of equal treatment between men and women in the access to and supply of goods and services,  OJ L 373/27.
 This meant, for example, that national law could allow car insurance companies to impose higher premiums on men than on women given that, in accordance with statistical data, men have a higher risk of causing a car accident than women.
 Case C‑236/09 Association belge des Consommateurs Test-Achats and Others, judgment of 1 March 2011, not yet reported, para. 20. See, in this regard, K. Lenaerts, ‘The European Court of Justice and Process-Oriented Review’ (2012) 31 Yearbook of European Law 3.
 Ibid., para. 26.
 Ibid., para. 30.
 Ibid., para. 32.
 21 December 2012 is the date on which the decision to derogate from the principle of equal treatment between men and women laid down in Article 5(2) of Directive 2004/113 was to be reviewed by the Member State concerned.
 See, e.g., Case C‑149/10 Chatzi  E.C.R. I‑8489, para. 43; Case C‑12/11 McDonagh, judgment of 31 January 2013, not yet reported, para. 44.
 Joined Cases C-402/07 and C-432/07 Sturgeon and Others  E.C.R. I-10923, paras 48-61. See also, Joined Cases C‑581/10 and C‑629/10 Nelson and Others, judgment of 23 October 2012, not yet reported.
 Regulation No 261/2004 of the European Parliament and of the Council of 11 February 2004 establishing common rules on compensation and assistance to passengers in the event of denied boarding and of cancellation or long delay of flights, and repealing Regulation (EEC) No 295/91,  OJ L 46/1.
 Sturgeon and Others, aboven 14, para. 46.
 Ibid., para. 47.
 Ibid., para. 57.
 The approach followed by AG Sharpston is somewhat different. She concurred with the Court of Justice in acknowledging that if compensation to passengers whose flight is delayed were excluded, then it would be impossible to reconcile Regulation No. 261/2004 with the principle of equal treatment. Yet, unlike the Court of Justice, the Advocate General did not provide a particular time-limit after which passengers whose flight is delayed enjoy a right to compensation. In her view, ‘the actual selection of a magic figure is a legislative prerogative’. See Opinion of AG Sharpston in Sturgeon and Others, above n 14, paras 93-94. However, the Court of Justice deployed another argument in order to counter this ‘separation of powers’ objection. It invoked Recital 15 in the preamble of Regulation No. 261/2004, whereby ‘the legislature (…) linked the notion of ‘long delay’ to the right to compensation’. Thus, the Court of Justice did not encroach upon the prerogatives of the EU legislator but simply limited itself to clarifying a legislative choice already contained in Regulation No. 261/2004, namely the distinction between ‘delay’ (inferior to three hours) and ‘long delay’ (equal to or in excess of three hours). Whilst the latter gives rise to compensation, the former does not. See Sturgeon and Others, above n 14, para. 62.
 Sturgeon and Others, aboven 14, para. 67 (extraordinary circumstances are defined as those which ‘are beyond the air carrier’s actual control’).
 Case C-281/98 Angonese  E.C.R. I-4139, paras 30-36.
 Case 43/75 Defrenne  E.C.R. 455.
 Case C-555/07 Kücükdeveci  E.C.R. I-365.
 Ibid., para. 25.
 Ibid., para. 31.
 Ibid., para. 46 (quoting Case 152/84 Marshall  E.C.R. 723, para. 48; Case C‑91/92 Faccini Dori  E.C.R. I‑3325, para. 20; and Joined Cases C‑397/01 to C‑403/01 Pfeiffer and Others  E.C.R. I‑8835, para. 108).
 Kücükdeveci, above n 23, para. 48 (referring to Case 14/83 von Colson and Kamann  E.C.R. 1891, para. 26; Case C‑106/89 Marleasing  E.C.R. I‑4135, para. 8; Faccini Dori, above n 26, para. 26; Case C‑129/96 Inter-Environnement Wallonie  E.C.R. I‑7411, para. 40; Pfeiffer and Others, above n 26, para. 110; and Joined Cases C‑378/07 to C‑380/07 Angelidaki and Others  E.C.R. I‑3071, para. 106.
 Kücükdeveci, above n 23, para. 49.
 Case C-144/04 Mangold  E.C.R. I-9981, paras 74 to 76.
 Kücükdeveci, above n 23, para. 50.
 Ibid., para. 51
 Opinion of AG Ruiz-Jarabo Colomer of 6 May 2004 in Pfeiffer and Others, above 26.
 Opinion of AG Ruiz-Jarabo Colomer in Joined Cases C-55 and 56/07 Michaeler & Others  E.C.R. I-3135, paras 20-22.
 Opinion of AG KokottinCase C‑321/05 Kofoed  E.C.R. I‑5795, para. 67.
 Opinion of AG Mazák in C‑411/05 Palacios de la Villa  E.C.R. I‑8531, paras 133- 138.
 Ibid., paragraph 138 (Article 19 TFEU states that the Council ‘may take appropriate action to combat discrimination’. It follows that it is for the Council to decide whether such action requires the adoption of a Regulation or a Directive). The same argument was deployed by the Court of Justice to reject the horizontal direct effect of directives in Faccini Dori, above n 26. There it held that ‘[t]he effect of extending that case-law to the sphere of relations between individuals would be to recognize a power in the [Union] to enact obligations for individuals with immediate effect, whereas it has competence to do so only where it is empowered to adopt regulations’. Ibid., para. 24.
 Opinion of AG Trstenjak in Case C-282/10 Dominguez, judgment of 24 January 2012, not yet reported.
 Ibid., para. 157.
 Opinion of AG Bot in Kücükdeveci, above n 23, para. 34.
 S. Prechal, ‘Competence Creep and General Principles of Law’ (2010) 3 Review of European Admnistrative Law 5, at 18-19.
 K. Lenaerts and J.A. Gutiérrez-Fons, ‘The constitutional allocation of powers and general principles of EU law’ (2010) 47 Common Market Law Review 1629, at 1647 et seq.
 Defrenne, above n 22.
 Angonese, above n 21.
 Defrenne, above n 22, para. 39.
 OJ  L 45/19.
 Defrenne, above n 22, para. 60. See also Case 96/80 Jenkins  E.C.R. 911, paragraph 22 (holding that Directive 75/117 could not alter the content or scope of that general principle); Case C‑17/05 Cadman  E.C.R. I‑9583, para. 29.
 OJ  Spec. Ed. Séries I-475.
 Angonese, above n 21, paras 34-35.
 For example, the Court of Justice observed that, while ex Article 119 EEC was formally addressed to the Member States, this circumstance did not exclude ‘rights from being conferred on individuals’ who seek to enforce the duties laid down therein. Ibid., para. 31.
 P. Craig and G. De Búrca, EU Law: text, cases and materials, 4th edn (Oxford, OUP, 2007) 276-277.
 Defrenne, above n 22, paras 19-22.
 Kücükdeveci, above n 23, paras 50-55.
 See K. Lenaerts and P. Van Nuffel, above n 5, 161, § 7-055.
 Case C-85/96 Martínez Sala  E.C.R. I-2691, para. 61. Lawful residence in the territory of another Member State suffices. It is not required that the citizen concerned has actually moved from one Member State to another: see Case C-148/02 Garcia Avello  E.C.R. I-11613, paras 13 and 27; Case C-200/02 Zhu and Chen  E.C.R. I-9925, para. 23.
 Case C-403/03 Schempp  E.C.R. I-6421, paras 22-25.
 See K. Lenaerts and P. Van Nuffel, above n 5, 162, § 7-055. See also K. Lenaerts, ‘European Union Citizenship, National Welfare Systems and Social Solidarity’ (2011) 18 Jurisprudencija 397.
 Case C-73/08 Bressol and Others  E.C.R. I-2735.
 Those 9 programmes were: Bachelor in physiotherapy and rehabilitation; Bachelor in veterinary medicine; Bachelor of midwifery; Bachelor of occupational therapy; Bachelor of speech therapy; Bachelor of podiatry-chiropody; Bachelor of physiotherapy; Bachelor of audiology; Educator specialised in psycho-educational counselling.
 Opinion of AG Sharpston in Bressol and Others, above n 59, para. 25.
 Bressol and Others,above n 59, paras 28-29.
 Ibid., para. 46.
 Ibid., para. 50.
 Ibid., para. 58.
 Ibid., para. 69.
 For the application of that framework of analysis by the Belgian Constitutional Court, see K. Lenaerts, ‘How the ECJ thinks: A Study on Judicial Legitimacy’ (2013) 36 Fordham International Law Journal (forthcoming).
 In addition, Article 9 TEU proclaims the general principle of the equality of citizens of the Union before the Union’s institutions, bodies, offices and agencies (principle of democratic equality).
 Council Directive 2000/43/EC of 29 June 2000 implementing the principle of equal treatment between persons irrespective of racial or ethnic origin,  OJ L180/22.
 Council Directive 2000/78/EC of 27 November 2000 establishing a general framework for equal treatment in employment and occupation,  OJ L303/16. See also Decision No 1672/2006/EC of the European Parliament and of the Council of 24 October 2006 establishing a Community Programme for Employment and Social Solidarity – Progress,  OJ L315/1.
 See K. Lenaerts and P. Van Nuffel, above n 5, 167, § 7-059.
 See the Community action programme to promote organisations active at European level in the field of equality between men and women, established by Decision No 848/2004/EC of the European Parliament and of the Council of 29 April 2004,  L157 (republished with corrigendum:  L195/7).
 See K. Lenaerts and P. Van Nuffel, above n 5, 168 and 169, § 7-059.
 Chatzi, above n 13.
 The framework agreement on parental leave concluded by UNICE, CEEP and the ETUC,  OJ L 145/4, as amended by Council Directive 97/75/EC of 15 December 1997,  OJ L 10/ 24.
 Chatzi, aboven 13, para.61.
 Ibid., para. 67
 Ibid., para.75.
 Case C-101/08 Audiolux and Others  E.C.R. I-9823 (ruling that that the alleged principle of equal treatment of minority shareholders could not be recognised as a general principle of EU law).
 Case C-174/08 NCC Construction Danmark  E.C.R. I-10567 (holding that the general principle of equality read in the context of the common VAT system only required Denmark to treat comparable economic operators alike).
 For a detailed analysis of those two judgments, see K. Lenaerts and J.A. Gutiérrez-Fons, above n 43, at 1660 et seq.
 Joined Cases C-22/08 and C-23/08 Vatsouras and Koupatantze  E.C.R. I-4585, para. 52.
 The essential conditions for a third-country national to qualify as a long-term resident are legal and continuous residence for five years, availability of sufficient resources and sickness insurance.
 Council Directive 2003/109/EC of 25 November 2003 concerning the status of third-country nationals who are long-term residents,  OJ L 16/44.
 Article 7(1) of Directive 2003/109 reads as follows:
‘1. Long-term residents shall enjoy equal treatment with nationals as regards:
(a) access to employment and self-employed activity, provided such activities do not entail even occasional involvement in the exercise of public authority, and conditions of employment and working conditions, including conditions regarding dismissal and remuneration;
(b) education and vocational training, including study grants in accordance with national law;
(c) recognition of professional diplomas, certificates and other qualifications, in accordance with the relevant national procedures;
(d) social security, social assistance and social protection as defined by national law;
(e) tax benefits;
(f) access to goods and services and the supply of goods and services made available to the public and to procedures for obtaining housing;
(g) freedom of association and affiliation and membership of an organisation representing workers or employers or of any organisation whose members are engaged in a specific occupation, including the benefits conferred by such organisations, without prejudice to the national provisions on public policy and public security;
(h) free access to the entire territory of the Member State concerned, within the limits provided for by the national legislation for reasons of security.’
 Article 7(2), 7(3), 7(4) and 7(5) of Directive 2003/19 read as follows :
‘2. With respect to the provisions of paragraph 1, points (b), (d), (e), (f) and (g), the Member State concerned may restrict equal treatment to cases where the registered or usual place of residence of the long-term resident, or that of family members for whom he/she claims benefits, lies within the territory of the Member State concerned.
3. Member States may restrict equal treatment with nationals in the following cases:
(a) Member States may retain restrictions to access to employment or self-employed activities in cases where, in accordance with existing national or Community legislation, these activities are reserved to nationals, EU or EEA citizens;
(b) Member States may require proof of appropriate language proficiency for access to education and training. Access to university may be subject to the fulfilment of specific educational prerequisites.
4. Member States may limit equal treatment in respect of social assistance and social protection to core benefits.
5. Member States may decide to grant access to additional benefits in the areas referred to in paragraph 1.
Member States may also decide to grant equal treatment with regard to areas not covered in paragraph 1.’
 Case C-571/10 Kamberaj, judgment of 24 April 2012, not yet reported.
 See K. Lenaerts and P. Van Nuffel, above n 5, at 170, § 7-060.
 See Case T-58/08 P Roodhuijzen  E.C.R. II-B-1-633, paras 68-102 (on medical insurance). See also Case C-485/08 P Gualtieri v Commission  E.C.R. I-3009, paras 70-76 (marital status considered relevant factor for determining amount of daily subsistence allowance).
 Directive 2004/38/EC of the European Parliament and of the Council of 29 April 2004 on the right of the citizen of the Union and their family members to move and reside freely within the territory of the Member States,  O.J. L158/77.
 Case T-307/00 C v Commission  E.C.R. – SC II-221, paras 48-56.
 Case C-267/06 Maruko  E.C.R. I‑1757, and Case C-147/08 Römer, judgment of 10 May 2011, not yet reported.
 Case C-167/12 C.D. (pending).
 Case C-363/12 Z v A (pending).