From microbes to humans, the natural world is full of spectacular examples of social behaviour. In the 1960s, W. D. Hamilton introduced three key innovations—now known as Hamilton’s rule, kin selection, and inclusive fitness—that changed the way we think about how social behaviour evolves, beginning a research program now known as social evolution theory. This is a book about the philosophical foundations and future prospects of that program. Part I, ‘Foundations’, provides a philosophical analysis of Hamilton’s core ideas, with some modifications along the way. We will see that Hamilton’s rule provides a compelling way of organizing our thinking about the ultimate causes of social behaviour; and we will see how, in inclusive fitness, Hamilton found a fitness concept with a special role to play in explaining cumulative adaptation. Part II, ‘Extensions’, shows how these ideas, when extended in certain ways, can help us understand cooperation in micro-organisms, cooperation among the cells of a multicellular organism, and culturally evolved cooperation in the earliest human societies. In all these cases and more, living things cooperate because they are related, where the concept of relatedness picks out relevant statistical patterns of similarity in the transmissible basis (genetic or otherwise) of social traits.