Recent research has shown that individuals are prepared to incur costs to punish non-cooperators, even in one-shot interactions. However, why would people punish non-cooperators with no apparent benefits for the punishers themselves? This behavior is also known as altruistic punishment. When defection is discovered, an individual evaluates this act as unfair, which could result in anger. We argue that although unfairness and anger are often intertwined, it is primarily the experience of anger and not the perception of unfairness that produces altruistic punishment. We briefly present recent data in line with the hypothesis that identifies anger as the underlying mechanism of altruistic punishment. Furthermore, additional influences regarding the occurrence of altruistic punishment, e.g., intentionality of the interaction partner, the role of satisfaction, and individual differences, are discussed. © 2009 New York Academy of Sciences.