This thesis proposes to open the EU's institutional black box. it examines how the EU's response to Switzerland can be explained by analyzing its inter-institutional interactions (i.e. Commission and Council). It attempts to ascertain which of these two institutions was the main “driver” behind the EU's positioning during the negotiations with Switzerland. To analyze this question, the thesis draws extensively on the principal-agent theory. Two competing hypotheses derived from intergovernmentalism and neofunctionalism are also developed. The first emphasizes the Council as a driver in the negotiations, while the latter puts the emphasis on the Commission. The empirical part mainly relies on written sources, including around 90 documents from the EU and the Swiss archives. The thesis finds evidence that the EU's initial response to Switzerland was shaped by the Commission. However, in later key moments of the negotiations, the Council had a crucial role, for instance in the agenda-setting phase