Although social policy was once perceived to be solely within the purview of the nation state, there has been a move toward a more European social policy. The European Court of Justice for the European Communities (‘Court of Justice’ or ‘Court’) determines the scope of European law and how it affects national welfare states. The court’s decisions will affect not only the national law of the member states with regard to social policy but also the direction of European social policy as it expands. However, the ECJ does not choose the policy areas in which it makes its decisions, but instead reacts to the preliminary references that are sent by the national courts of the Member States. These preliminary references from the Member States will set the Court’s agenda. Preliminary references are unevenly distributed across the Member States of the EU, and some Member States’ preliminary references are concentrated in particular policy areas. The jurisprudence of the Court, and consequently the social policy of the EU, could be steered by this uneven distribution. This thesis will answer the threshold question of why scholars of social policy should care about the Court of Justice, with a legal analysis of some key themes in the Court’s decisions in the area of social policy. It will then employ a mixed methods research design to explain the variation in rates of social policy preliminary references from the EU-15. First, a Time Series Cross-Section (TSCS) model will be used to test a series of hypotheses generated from the literature, and three novel hypotheses, in a dataset of social policy preliminary references from the EU-15 from 1996 to 2009. Next, a Qualitative Comparative Analysis (QCA) (Ragin 2000) will group the variables that were found to be significant into sets of conditions, or ‘causal pathways,’ that lead to higher and lower rates of social policy preliminary references. Finally, two qualitative case studies will be conducted, in the UK and France. Analysis of documentary evidence and 25 expert interviews in the two member states and at the Court of Justice will further explain and illuminate the differing usage of preliminary reference process. The analysis of the mixed methods is integrated in the final stage. Implications for the direction of EU law related to social policy and the future development of European social policy will be considered in the concluding chapter.