The current use of fossil fuels for the purpose of energy and materials production creates a number of pressing sustainability problems, such as climate change. Therefore, the fossil fuel system is in need of a fundamental transformation. However, despite this need, the targets set by policy makers, and the availability of renewable energy technologies, the fossil fuels system keeps expanding. This raises the question of why it is so difficult to move away from fossil fuels. To answer this question, this thesis focuses on both institutions and incumbents in the energy transition. The fossil fuel system is aligned with an extensive set of rules or institutions that both enable and constrain our behavior and provide meaning to our society. An important characteristic of institutions is that they provide stability and resist change. Incumbents have heavily invested in the fossil fuel system and therefore have extensive vested interests. Incumbents stand to lose a large share of their potential profits when energy production shifts towards renewable energy technologies. Therefore, it is likely that incumbents will attempt to protect their fossil fuel interests. The field of sustainability transitions has acknowledged the central role of institutions and the importance of incumbents in transition processes. However, little attention has been paid to the relationship between incumbents and institutions. This thesis focuses on how incumbents influence institutions as well as on how institutions influence incumbents. The theoretical framework consists of the institutional work and institutional logics stream within institutional theory. This thesis contains four case studies in the context of the Dutch fossil fuel system. The Netherlands was chosen for its stagnant energy transition and its many large incumbents related to fossil fuel activities. Our data collection includes interviews, newspaper articles, policy documents, annual reports, and organization websites. Analysis was conducted through categorization and ...