This book outlines four different categories of co-operation — reciprocal altruism, kinship, group-selected co-operation, and by-product mutualism — and ties them together in a single framework called the Co-operator's Dilemma. Hundreds of studies on cooperation in insects, fish, birds, and mammals are then reviewed, each example being tied back to the theoretical framework developed early on when the data allows. Future experiments designed to further elucidate a particular type of co-operation are provided throughout the book.