This chapter analyses the determinants that lead national courts across EU countriesto use the preliminary reference procedure, paying special attention to the differences andsimilarities in the use of this mechanism of judicial cooperation between the old and thenew Member States incorporated in 2004 and 2007. The study presents original andcomprehensive data on the use of preliminary references (1961-2011) in all 27 Memberstates. Besides confirming the impact of common factors already tested in the literature,this research additionally identifies some differences in the institutional dynamicsinfluencing the use of preliminary references across older and newer Member States.