A key principle of EU law is that of direct effect. The now CJEU recognized it for the first time in the landmark 1963 judgment in Van Gend en Loos (Case 26–62, EU:C:1963:1).

Insofar as EU directives are concerned, the dominant view is that they do not produce any horizontal direct effect. This means that they cannot be relied upon in private-party proceedings and do not impose obligations on individuals. Nevertheless, EU directives may produce—at certain conditions—a vertical direct effect. In cases in which EU law imposes upon Member States ‘the obligation to pursue a particular course of conduct, the useful effect of such an act would be weakened if individuals were prevented from relying on it before their national courts and if the latter were prevented from taking it into consideration as an element of [EU] law’ (van Duyn, Case 41–74, EU:C:1974:133, para 12).