The 37 EU agencies are a sizable part of the EU’s multi-level administration. Created in negotiations between the European Commission, the European Parliament and the Council of the European Union, these semi-independent bodies contain intergovernmental, supranational and transnational characteristics. EU agencies are networked bodies: not only the European Commission, but also national authorities are omnipresent on their management boards and in their working groups. Because of their voting majority on management boards and in EU agencies’ working groups, national officials’ behaviour can determine EU agencies’ output. While the Commission pursues a clear supranational agenda, the role of national representatives remains ambiguous: Often detached from direct domestic political control and – at least in theory – accountability-holders of EU agencies, national representatives have incentives to control EU agencies, align their work with their domestic one or co-create EU-level solutions. This thesis addresses the question, when the behaviour of national officials promotes intergovernmental, bureaucratic or transnational interests in EU agencies. Empirically, the thesis proceeds in three steps. First, it develops a typology of EU agencies according to their main activities and classifies all 37 agencies into four categories: authorisation, regulation, implementation and information agency. Second, it maps the formal, de jure role of national representatives in all 37 agencies’ governance structure on the basis of their founding regulations. This mapping exercise shows that the stronger the formal competences of an agency, the stronger is the position of national representatives in an agency’s governance set-up. Moreover, the mapping reveals variation across the four agency types. Building upon these findings, the study selects four EU agencies that cover the variance in formal governance set-ups for in-depth case studies on national representatives’ behaviour: the European Environment Agency as an information agency, the European Maritime Safety Agency as an authorisation agency, the European Maritime Safety Agency as an implementation agency and the European Banking Authority as a regulatory agency. On the basis of 47 semi-structured interviews with 53 national and Commission representatives as well as agency officials, the final part of the study illustrates that different governance set-ups have only a mediating influence on the interests, national representatives promote in EU agencies. Across the entire variance of formal structures, national representatives safeguard their bureaucratic interest, especially where they risk being overburdened by the administrative consequences of EU agency decisions. Only in salient agencies, intergovernmental control concerns play a significant role. Primarily, however, national representatives create synergies between their domestic work and EU agencies’ work. Even though this is likewise motivated by bureaucratic interests, it contributes to the expansion of EU agencies tasks and a further centralisation of the EU’s multi-level administration. ii Moreover, national representatives value the learning opportunities that EU agencies provide and in case of different opinion with the Commission or other national representatives prefer to invest time in finding consensual solutions instead of blocking further action. Thereby, national representatives across all four cases do not only represent exogenous, domestic preferences but also transnational interests, endogenous to the specific environment around each EU agency. These findings indicate that national representatives both constrain and enable EU agencies. Their bureaucratic turf and reputational interests set the boundary at which they begin to oppose further EU agency measures. Their interest in efficient workflows and coordination as well as their genuine interest in EU-wide solutions, on the other hand, enables EU agencies to perform and occasionally expand the scope of their actions.