This project will put a spotlight on joint bodies (JBs), which refers to common institutions set up between the European Union (EU) and third countries through their bilateral agreements. They bring together representatives from the EU (Commission and/or Council) and the third country to oversee implementation. Beyond the ability to discuss common problems and issue recommendations, they are sometimes even empowered to amend the agreement or adopt binding decisions, which raises concerns about their democratic legitimacy. Although the number of JBs run into the hundreds and their decision-making powers attract increasing attention, our knowledge about them is limited. This project would help to significantly narrow this discrepancy. The objective of this project is to explain Commission discretion in these bodies. We draw on principal–agent theory to highlight that, in designing international institutions, states rationally weigh expected benefits of delegation against expected costs. We argue that the size of agency losses is determined by whether agreements are concluded in an area of high politics, the need for policy expertise, the duration of an agreement, and the underlying decision rule among principals. In terms of third-country characteristics, we theorize that size and regime type (democracy) play a role in states’ determination of agency losses. To test these expectations, we will first generate an original dataset mapping JBs across ca. 550 bilateral agreements and primarily use regression analyses, albeit with some qualitative evidence from interviews to corroborate the causal mechanisms. Second, we will use stratified random sampling to conduct a survey among at least 100 Commission officials active in JBs and interpret the data employing both quantitative and qualitative methods. This project will be empirically innovative, theoretically informed, and methodologically rigorous and produce findings relevant for scholars, policy makers, and citizens.