Punishment has been regarded as an important instrument to sustain human cooperation. A great deal of experimental research has been conducted to understand human punishment behavior, in particular, informal peer punishment. What drives individuals to incur cost to punish others? How does punishment influence human behavior? Punishment behavior has been observed when the individual does not expect to meet the wrongdoers again in the future and thus has no monetary incentive to punish. Several reasons for such retributive punishment have been proposed and studied. Punishment can be used to express certain values, attitudes, or emotions. Egalitarianism triggers punishment when the transgression leads to inequality. The norm to punish the wrongdoers may also lead people to incur costs to punish even when it is not what they intrinsically want to do. Individuals sometimes punish wrongdoers even when they are not the victim. The motivation underlying the third-party punishment can be different than the second-party punishment. In addition, restricting the punishment power to a third party can be important to mitigate antisocial punishment when unrestricted second-party peer punishment leads to antisocial punishments and escalating retaliation. It is important to note that punishment does not always promote cooperation. Imposing fines can crowd out intrinsic motivation to cooperate when it changes people’s perception of social interactions from a generous, non-market activity to a market commodity and leads to more selfish profit-maximizing behavior. To avoid the crowding-out effect, it is important to implement the punishment in a way that it sends a clear signal that the punished behavior violates social norms.