This article examines the influence of Europeanization on the relationship between ministries and agencies at the national level. The core argument is that the differentiated nature of the international environment (with policy development often transferred to the international level and policy implementation left at the national level) transforms national agencies into policy-developing actors that shape policies without being directly influenced by their national political principals. The increasingly common involvement of national agencies in European policymaking processes thereby increases these agencies' policy-development autonomy but does not change their role in policy implementation. We examine this argument by testing an innovative hypothesis—the differentiation hypothesis—on a combined data set of German and Dutch national agencies. Our empirical findings support the hypothesis in both countries, suggesting that similar effects can be expected in other contexts in which semiautonomous agencies are involved in transnational policymaking.