This chapter proposes an inquiry into the surface of EU law. This implies a non-formalist study of legal forms such as legal sentences, concepts and techniques, a rich field of inquiry in itself. These are not mere tools, totally under control, used to achieve certain ends defined in other terms, outside the law. They are far more intriguing and fascinating than that. These legal forms participate to the constitution of legal and social realities that they pretend to regulate. They enable and limit what participants of a language-game can do, and they can even influence what they may want to do. The proposed methodology to study the CJEU implies focusing on law and legal concepts as a set of knowledge practices and inquiries on the transformative power of legal techniques. It should be distinguished from classical studies on the Court, mainly dealing with interpretation, as well as from works focusing on power relations between legal actors. Through the presentation of some cases in EU citizenship case-law, this chapter focuses on the constitutive dimension of the legal controversy surrounding the Court of Justice of the European Union’s rulings.