In order to fill the 'EU regulatory gap' caused by the mismatch between the single market programme and the lack of EU regulatory capacity, a number of EU regulatory agents were created. Committees, networks and EU agencies mushroomed in order to fulfill different regulatory functions. The thesis aims at explaining the variation of these delegation patterns between sectors and over time. Combining an innovative and refined functional-institutionalist approach and power-distributional factors, the thesis first argues that the distribution of implementing competences has a crucial effect on the delegation pattern. While nationally based implementation would explain the establishment of EU regulatory networks, expert committees would be found where most implementing competences are in the hands of the Commission. Second, the gradual reinforcement of networks and committees up to their possible transformation into EU agencies is addressed by a dynamic relationship between functional and distributional forces unfolding over time through feedback loops. Keen on keeping their power, policy-makers set up weak agents before expanding their power at a later stage after realizing they lacked the means to achieve the policy objectives assigned to them. The empirical analysis, based on three case studies (the regulation of food safety, electricity and telecommunications) confirms and completes the conjectures by pointing at additional factors such as the presence of independent regulatory agencies at the national level, the technicality of the sector and sociological pressure. In addition to providing a wealth of new insights on regulatory delegation in the EU, the thesis offers a sophisticated adaptation of the principal-agent framework in multiple principals configurations and makes a strong case for refining the conceptualization of functional pressure and colouring the study of institutional choice, otherwise dominated by distributional and institutional factors, with a revamped functional approach.