Buchanan believed that individuals are fundamentally willing to cooperate with others.It was at the center of his works in public finance in the late 1940s and early 1950s, and also crucial to his work in public choice in the 1960s.The purpose of this book is to show which forms this belief took over these two decades or so, and to explain the continuity between these forms.We adopt a historical approach that allows us to recount the story of how Buchanan came to develop a theory of collective action, including his conception of cooperation in small groups, to implement a technical condition about the pricing of public goods he defended early in his career.We describe the different steps Buchanan took, the encounters that influenced him, and the events and challenges that led him to revise his views to make room for this fundamental philosophical conviction.