Rationalist theories of international institutions dominated the theoretical debate in International Relations throughout the 1980s. Moreover, club theory, the general rationalist theory of the size of organizations, is the best developed and most pertinent approach to explaining enlargement. For these reasons, I begin my analysis of Eastern enlargement with rationalist institutionalism. In the theoretical chapter (chapter 1), I describe the basic assumptions of rationalist institutionalism, present club theory, distinguish a security, power, and welfare approach to enlargement, and specify the conditions of enlargement for each approach. In the empirical chapters, I check to what degree these conditions were fulfilled in the Eastern enlargement of NATO (chapter 3) and the European Union (chapter 2). However, I will conclude that, whereas rationalist institutionalism accounts for the Central and Eastern European interest in joining the Western organizations, it does not convincingly explain why the EU and NATO member states agreed to expand their organizations.