The early years of life offer ample opportunities for revenge, as young children face frequent frustrations and often use physical force against others. Still, studying revenge in young children is challenging, theoretically and methodologically. This chapter proposes criteria for studying revenge, synthesizes what is known about the origins of revenge, and points to new questions for research. We define revenge as an effort to harm someone who has affected you negatively. Based on extant evidence, we conclude many of infants’ aggressive responses likely constitute early forms of revenge. Later, from around the third birthday, children also appear capable of taking revenge against someone who has wronged them. From that age, children can also form moral judgments about revenge; both children and adults judge that revenge is sometimes permissible. Throughout the chapter, we stress the need to distinguish revenge from other forms of reactive aggression in future research.