Niamh Nic Shuibhne has tackled an important issue that arises frequently within discourse concerning free movement law, which is the coherence of the complex case law developed by the ECJ since the inception of the EEC. The study is grounded on the assumption that the ECJ operates in the mode of a constitutional court and has the responsibilities associated with such judicial organs. Coherence is not felt to demand that different sectors must be treated in exactly the same legal manner, but rather that differences are justified by principled distinctions as between the application of free movement principles in different areas. Niamh Nic Shuibhne contends that four main factors have distorted the meaning of free movement case law, and led to fragmentation of the case law, irrespective of whether these factors reflect good impulses or not. They are the protection of fundamental rights, the proliferation of principles, the contested purpose of the internal market and free movement, and the structure of the Court and its decision-making processes. These problems are felt to be exacerbated by the Court’s reluctance to engage with or reverse its previous decisions, even where it seems to be departing from them. The discussion is organised thematically, focusing on the principles that drive the developing case law. It is framed so as to consider in turn the negative and positive scope of free movement law. The former connotes the principles that are applied in free movement law to exclude something/someone from the scope of the Treaty, thereby rendering recourse to issues of justification and proportionality redundant. This includes issues such as the horizontal reach of free movement provisions, and the status of the rule that wholly internal situations are not covered by free movement law. The latter, the positive scope of free movement law, is concerned with the principles that determine the outer reach of the legal rules in this area. The coverage focuses on the utility of the division between directly and indirectly discriminatory restrictions, and also the much discussed issue concerning the extent to which EU law should cover non-discriminatory measures.