How does policy implementation impact on the institutionalised capacities of political agents? This question about the link between policy-making and institutional development lies at the heart of the study. It is inspired by an empirical puzzle about the dynamics of European integration. Since 1993 the European Union has developed a strategic approach to enlargement the most relevant trademark of which has been the criteria which acceding states have to meet before entering the Union. The conditionality-based preaccession policy provided the European Commission with a set of new competences vis-à-vis the candidate states that were not directly derived from the internally applicable legal framework, the acquis communautaire. This kind of conditionality established double standards applicable to the candidate, but not the member states. Scrutinising all cases in which such double standards were created brings to the fore the fact that only in two cases did policies remain indeed limited to the candidate states before accession. In a further three cases the steering instruments the Commission developed in the enlargement context were extended beyond the institutional sub-system of the pre-accession policy to the institutional core of the acquis: why and how? The theoretical framework draws from functionalist theory. The necessary condition for policies to be integrated from a distinct institutional sub-system – such as the pre-accession policy that applied to candidate states only – is functional pressure. Double standards create functional pressure for integration per se because they undermine the credibility of the political system in that candidates for membership need to comply with higher standards than members themselves. Functional pressure rises if policies are implemented successfully and if political problems persist so that at the moment of accession they become a common matter of concern for all member states. However, this alone does not explain why a single policy ‘spills-in’ from a sub-system to the institutional core. The sufficient condition and theoretical explanation builds on Theodore Lowi’s arenas of power approach. The variance between the single policies is explained by the policy type at stake that determines which steering instruments emerge in the respective arenas of power. The empirical results show that the Commission indeed extends its steering capacity if policies are formulated as non-binding rules and standards in the regulatory arena or in a restrictive way that limits redistributive effects in the distributive arena. In conclusion, the thesis contributes to three strands of European integration research. First, it offers a theoretically-guided analysis of European Union widening. Enlargement policy is conceptualised as an institutionally linked but distinct arena of institutional rules. The effect that policy-making in such an institutional sub-system has may hence be extended and applied to other institutional sub-systems that are linked to but distinct from the Union’s core legal framework. Second, the study provides insights into the functioning the European Commission and how the organisation continues effectively to extend its steering capacities in the post-Maastricht era. Moreover, the case studies cover policy fields not much discussed in the existing scholarly literature. They therefore provide original research on how the Commission actually develops new responsibilities and implements policies. Third, the findings inform the study of European integration at large. Spelling out how implementing enlargement has been a source for extending the European Commission’s action capacity, the thesis highlights the process of widening as a specific cause for supranational institutionalisation which has so far attained little if any attention.