How to design punishment mechanisms to maintain social order is a central question in law. Xiao reviews recent behavioral economic research on how punishment influences prosocial behavior, much of which provides evidence that punishment can have a negative effect on cooperation. She discusses several factors that can lead to the detrimental effect of punishment and suggests some solutions. She argues that the effectiveness of punishment is closely connected to its role in expressing social norms. She ends the chapter by reviewing some recent research on social norms and suggesting punishment and social norm instruments are complementary in promoting cooperation.