AbstractImitation is argued to have an important affiliative function in social relationships. However, children's tendency to imitate different play partners during naturalistic play and associations with social understanding have been overlooked. We investigated the frequency and context of imitation in a longitudinal study of 65 focal children (T1: M age = 56.4 months, SD = 5.71) during play with their older or younger sibling and a friend in two separate play sessions. Children were observed again approximately 3 years later (T2: n = 46, M age = 94.6 months; SD = 6.6). We coded focal children's verbal and nonverbal imitation of their play partner, their partner's response to being imitated, the context in which imitation occurred (e.g., pretense), and the focal child's social understanding (i.e., mental state references). Verbal imitation occurred more often than nonverbal imitation and was used most often during the contexts of play negotiations and pretense. Although focal children's imitation of both their siblings and friends increased significantly over time, children imitated friends more than siblings at T1. All play partners responded positively (i.e., smiling, laughing) most often to being imitated. Associations between focal child imitation and mental state talk with friends at T2 approached significance. Our findings provide a deeper understanding of the nature of imitation during children's play interactions and support assertions that imitation is a process whereby children build affiliation, mutuality, and shared meanings in their relationships.