Book excerpt: The "tragedy of the commons" is a central concept in human ecology and the study of the environment. It has had tremendous value for stimulating research, but it only describes the reality of human-environment interactions in special situations. Research over the past thirty years has helped clarify how human motivations, rules governing access to resources, the structure of social organizations, and the resource systems themselves interact to determine whether or not the many dramas of the commons end happily. In this book, leaders in the field review the evidence from several disciplines and many lines of research and present a state-of-the-art assessment. They summarize lessons learned and identify the major challenges facing any system of governance for resource management. They also highlight the major challenges for the next decade: making knowledge development more systematic; understanding institutions dynamically; considering a broader range of resources (such as global and technological commons); and taking into account the effects of social and historical context. This book will be a valuable and accessible introduction to the field for students and a resource for advanced researchers.