With the successful launch of the single currency the European Union (EU) is now focused intensely on the second great project of the post-Maastricht agenda - enlargement to Central and Eastern Europe (CEE). After a decade of lofty rhetoric and continued prevarication, the EU committed itself, at the Helsinki summit in December 1999, to a full and inclusive enlargement round. Given the sheer scale of the challenge this represents it should hardly be a surprise that this enlargement has inspired a steady stream of academic publications. But the vast majority of those publications have been empirical. This has meant that analysis of the enlargement process has, as Schmitter has suggested, been taking place in a “theoretical vacuum”. This paper represents an effort to redress the balance in favour of theoretical endeavour. The paper begins by assessing the relative merits of IR theories applied to the enlargement process. Insights from Neorealism, Neoliberal Institutionalism and Neofunctionalism are tested with respect to core propositions on enlargement. However, it emerges that enlargement represents a puzzle for all of these approaches in that, as Schimmelfennig has suggested, none of them can explain why a process characterized in its early stages by the rational pursuit of perceived interests by EU member states somehow has ended up in a normatively determined outcome with the decision taken by the EU at the Helsinki summit, to open negotiations with all of the candidate countries from CEE. The paper goes on to examine the claims of social constructivism as an alternative explanatory framework. In opposition to the methodological individualism and static conception of identity transformation in international politics offered by rationalists, constructivists emphasize the co-constitution of the material and social worlds and the significance of norms, rules, and values in the international arena. The EU as a densely institutalized environment seems a natural entity for the application of constructivist theory. With respect to the enlargement process this analysis suggests that the constitutive values of the European Union, predicated on normative understandings of what ‘Europe’ represents, and manifested in the Copenhagen criteria, represent the key building blocks for this enlargement round. This is not to deny the importance of material phenomena such as the ‘national interests’ of the member states. But it is to suggest that constructivism provides a much more nuanced explanation of the enlargement process.