The development of transnational political parties has been a consistent feature of West European politics in the post-war years. European integration has propelled the establishment of two different types of transnational party organisation: the transnational party federation and the political group in the European Parliament. Though it might be conventional wisdom to present regionalist parties as isolationist and hostile towards internationalism and European integration, the parties have combined to develop their own transnational federation and sought to establish a political group of regionalists in the European Parliament. Rather than remain aloof from international developments, regionalist parties in different parts of Europe have been intimately concerned with international issues and developments, and have shown great interest in European integration in particular. Part of these parties’ response to European integration has involved participation in European institutions and membership of transnational political organisations. This chapter will offer a historical assessment of the attitudes of regionalist parties towards transnational co-operation and analyse the development of the regionalists’ own transnational federation, the European Free Alliance (EFA). It will also offer an examination of the activities of regionalist parties in the European Parliament and their efforts to establish a regionalist political group within the parliament, such as the Rainbow Group from 1984 to 1994, to rival those of the other European political families.