The function of a social norm is to coordinate people's expectations in interactions that possess multiple equilibria. Norms govern a wide range of phenomena, including property rights, contracts, bargains, forms of communication, and concepts of justice. Norms impose uniformity of behaviour within a given social group, but often vary substantially among groups. Over time norm shifts may occur, prompted either by changes in objective circumstances or by subjective changes in perceptions and expectations. The dynamics of this process can be modelled using evolutionary game theory, which predicts that some norms are more stable than others in the long run.