This essay takes stock of the literature on how European Union policies are being put into practice by the member states. It first provides an overview of the historical evolution of the field. After a relatively late start in the mid-1980s, the field has meanwhile developed into one of the growth industries within EU research. The paper identifies four waves of EU implementation scholarship, each with its own theoretical, empirical and methodological focus. In the second part, the review discusses the most important theoretical, empirical and methodological lessons to be drawn from existing studies. Four conclusions emanate from the analysis of existing EU implementation research. First, the literature has focused heavily on the transposition of EU directives, while comparatively little is known about issues of enforcement and application of both directives and regulations or about member states' reactions to negative integration. Second, scholars studying the transposition of directives seem to agree that we need to address factors that influence member states' willingness and capacities to comply. The main task to be accomplished by future research is to establish under what conditions which configurations of factors prevail, especially with regard to sectoral differences. Third, more energy needs to be devoted to systematic research on the phase of practical implementation, and this research should make more use of theoretical insights from domestic implementation research as well as from management and enforcement approaches. Fourth, quantitative transposition research will have to improve the data it uses to measure the dependent variable. Scholars should explore better data sources and invest more energy in collecting their own data on transposition timing and correctness. Research on application and enforcement, on the other hand, needs to go beyond case studies and instead search for or produce data with which the practical phase of implementation can be analysed on a broader, more comparative scale.