How to produce and utilise knowledge in the best way for sustainable environmental governance is a pressing question of our time. It involves the assembling of credible expertise from different actors (scientists and other experts) in processes that are regarded democratically legitimate. However, these two governance components (expertise and legitimacy) can be in conflict, when science steered expert advice informs the governance of the commons. EU fisheries management, serving as empirical case in this book project, is a chief example of this tension. The book is divided into a conceptual part, introducing theoretical research developments about expertise and legitimacy in environmental governance, and an empirical part, which explores these issues in fisheries and marine governance. This domain still lacks a coherent contribution from a critical social science perspective that investigates the interaction and mutual dependencies of expertise and legitimacy in relation to different actors and their agency. The book aims to fill this gap and contributes to ongoing research debates for building long term sustainable resource management, that also includes the social aspects of sustainability (equity, social justice, distribution of power and participation). It also draws lessons for environmental governance and sustainably science generally. The project includes research visits to the universities of Wageningen (fall 2023) and Tromsø (spring 2024).