Game theory in biology seeks to predict social behaviour and other traits that influence how individuals interact. It does this by tentatively assuming that current traits are stable endpoints of evolution by natural selection. The theory is used to model aggressive behaviour, cooperation, negotiation, and signalling, as well as phenotypic attributes like an individual’s sex and mating type. This book covers the basic concepts and the traditional examples of biological game theory. It expands the frontiers of the field, emphasizing the importance of the co-evolution of traits and the implications of variation for reputation, markets, negotiation, and other social phenomena. It also highlights that it can be important to embed game interactions in the environment and an individual’s life. A major new direction developed in the book is that game theory can be extended by incorporating behavioural mechanisms, including mechanisms of reinforcement learning. By doing this the theory can successfully describe important phenomena like social dominance in group-living animals that previously have been difficult to model. By focusing on behavioural mechanisms, game theory can also make closer contact with empirical observation and with current research in fields like animal psychology and neuroscience.