A comprehensive philosophical theory of co-operation, this text builds on such notions a collective and joint goals, mutual beliefs, collective commitments, acting together and acting collectively. The book analyzes the varieties of co-operation, making use of the crucial distinction between group-mode and individual-mode co-operation. The former is based on collective goals and collective commitments, the latter on private goals and commitments. It discusses the attitudes and the kinds of practical reasoning that co-operation requires and investigate some of the conditions under which co-operation is likely, rationally, to occur. It also shows some of the drawbacks of the standard game-theoretical treatments of co-operation and presents a survey of co-operation research in neighbouring fields.