Abstract: This chapter focuses on the evolution of strong reciprocity. A predisposition to cooperate and a willingness to punish defectors is known as strong reciprocity, and it is the combination of the two that is essential to human cooperation. Punishment reduces the gain to free-riding, and may induce even entirely self-interested individuals to cooperate. Before explaining how a willingness to punish those who violate social norms even at personal cost could have evolved, the chapter considers how punishment is coordinated among group members so that it is contingent on the number of others predisposed to participate in the punishment. It also shows that punishment is characterized by increasing returns to scale, so the total cost of punishing a particular target declines as the number of punishers increases. Finally, it discusses the results of simulations that illustrate the emergence of strong reciprocity and examines why coordinated punishment succeeds.