This book systematically describes and evaluates the impact of energy cooperatives as a key driving force in the German energy transition toward a sustainability-oriented energy sector. Based on a comprehensive survey and three case studies, it provides an instructive overview of the overall dimensions and scope of energy cooperatives in Germany, and of their history, structure and current investment projects. The book not only contributes to the energy policy discourse in Germany, but also highlights the role of energy cooperatives to enable an international readership to explore their potential in other countries. Further, it makes a theoretical contribution toward substantially supplementing actor research in general, and enterprise research in particular, in the field of sustainability transitions science.