This PhD thesis is about the involvement of EU agencies and EU networks of national authorities in the enforcement of EU policy. For several decades the EU's member states were the primary enforcers of EU regulation vis-à-vis citizens and companies. Increasingly, however, EU policies are also being enforced by EU agencies and EU networks of national authorities. This thesis revolves around the question: why do EU legislators establish these EU agencies and EU networks of national authorities for the enforcement of EU policies, and why do they differentiate between them? This question is answered through case studies of enforcement in four policy areas: the regulation of civil aviation airworthiness, civil aviation incident investigation, the regulation of medical devices, and the regulation of medicines. This multidisciplinary and comparative thesis relies on insights from both political sciences and legal academic literature, and leads to more insight in the enforcement of EU policy, the politics of the EU, and the EU in its entirety.