This dissertation examines the enlargement of the European Union (EU) as a process of policy evolution and change. It answers whether and how, when new members join the EU with distinctive preferences, practices and traditions, these are accommodated into EU policy, by being inserted (policy change) or by having to adapt (no policy change). The evidence provided derives from the accessions of Spain and Finland to the EU's trade and foreign policies. The central argument is that when new members join the EU, their accession sets in motion a dynamic process that is not simply a reflection of internal and external forces, but one that we need to analyze on its own. Beginning with the conflicts between existing and new preferences, and the actions/interactions by the newcomers to insert them with different degrees of constraint, I argue that it is sometimes possible for newcomers to alter the existing policy trajectory. Successful insertion, which is more likely when newcomers use opportunities to consolidate advantage rather than to challenge existing institutions, allows for the reproduction of the trajectory along those same lines. Over time, this process stabilizes the policies while subtly transforming them. This approach contrasts with most analyses of enlargement as exogenous forces disconnected from the integration process. By focusing more closely on the interactions between the external and internal processes, the present study explains better the effects that successive enlargements have had on the integration process. Theoretically, the study contributes to the institutionalist debate on the EU and on international institutions by developing more specific propositions as to whether and how policies evolve over time in particular contents and forms and not in others. Practically, it provides insights for the policy-makers of the more recent and upcoming accessions, as to the circumstances in which their preferences are more likely to be taken into account.