The Summer School on Energy Systems will focus on the economic, policy and regulatory aspects of sector coupling in the European Union and the possible ways to model and investigate them quantitatively. In the first part of the course, participants gather a general overview of European energy policy and the importance of coupling different energy end-uses and vectors to achieve the energy transition. Given their central role in the current debate and business developments, the organisation of the electricity and gas markets, the regulation of the related networks, and their interplay will be presented. In the second part of the course, participants explore a few quantitative studies and modelling exercises that shed some light on the problems and issues identified in the first part. Participants have the opportunity to present their current research on the topics and receive accurate feedback from peers and the course faculty.