Since California's formative years during the Gold Rush, its development has intertwined with its water law. Using the state's constitutional water policy and that policy's history as lenses, this book examines the many rules and doctrines that govern the use of what is often called the state's most precious resource. This book explains the many kinds of water rights, how they interact, how they can be modified and how they can be lost. It describes how environmental interests affect water rights' exercise and the continuing questions those interests raise. By describing not only the many relevant laws, but also the reasons for them, this book will help judges, students, attorneys and other professionals to understand both the what and the why of this important field.