The Free Movement Directive is the key EU legal instrument that regulates the free movement of persons within the thirty-one countries that make up the European Economic Area. Implementation of Directive 2004/38 by the Member States has been far from satisfactory and the Commission has deplored the fact that not a single Member State had been able to implement the Directive correctly. The enforcement action that the Commission has so far taken does not appear to have necessarily been linked to how well transposition complies with the Directive in individual Member States. In other words, the way in which the Directive has been written into national law does not seem to be the only factor that determines how well that Member State might implementing the Directive. The purpose of this study is to investigate the implementation gap in the context of Directive 2004/38. This study is primarily concerned with the practical implementation of EU law in the Member States in the specific area of the free movement of persons. This dissertation seeks to engage with the established theoretical legal scholarship on the free movement of persons, as well as with ongoing discussions that continue to animate empirical research on the implementation of directives. The research is intended to make an original contribution in the field of implementation research by focusing on all three stages of implementation of EU directives - transposition, application, and enforcement - which has so far been neglected by the majority of studies that engage in empirical research on the implementation of directives. This study pursues a two-fold aim. Firstly, it aims to identify the various elements of implementation that should be taken into account to assess the overall implementation of the Free Movement Directive both in law and in practice. More specifically, the first purpose of this research is to investigate how implementation may be measured as a dependent variable in a way that goes beyond transposition so that it also encapsulates ...