In recent decades, cross-national policy clustering has become a distinctive feature of international and European environmental policy-making. Since the late 1960s, virtually every country in the world has created government institutions for the protection of the environment, such as environmental ministries, national environmental agencies or environmental advisory councils and basic legislation in the areas of air pollution control, nature and water protection as well as waste management has successively been adopted in a large number of countries. This trend is even more pronounced if one focuses only on the smaller and more homogeneous group of European Union (EU) Member States. Here, over the last thirty years, an impressive convergence of domestic patterns of environmental policy-making can be observed. Can a similar degree of policy clustering also be observed with regard to programs aimed at the promotion of renewable energies (RES)? And if so, what are the mechanisms that drive the EU-wide convergence of these programs? In order to provide an answer to these questions, this chapter examines the cross-national spread of support schemes for electricity generation from RES sources, namely mandatory feed-in tariffs and green certificate systems, in the period from 1988 to 2005. Based on an analytical framework that distinguishes between three broad groups of mechanisms of cross-national policy coordination – cooperation, coercion, and diffusion – the chapter explores the main driving forces as well as the barriers of a greater promotion of electricity from RES sources in the EU Member States. It finds that diffusion – i.e. processes of voluntary imitation and learning among governments – has played a major role in the Europeanisation of domestic renewable energy policies. Based on this empirical finding, it is argued that diffusion as a distinct Europeanisation mechanism should receive more attention in future research.