Social identification is known to have wide-reaching implications, but theorists disagree about the underlying motives. Integrating motivated identity construction theory with recent social identity research, the authors predicted which motives underlie identification with two types of groups: interpersonal networks and social categories. In a five-wave longitudinal study of social identity processes among 268 new university residents, multilevel analyses showed that motives involved in identity enactment processes—self-esteem, belonging, and efficacy—significantly predicted within-person changes in identification with flatmates (an interpersonal network group), whereas motives involved in identity definition processes—meaning, self-esteem, and distinctiveness—significantly predicted within-person changes in identification with halls of residence (an abstract social category). This article discusses implications for research into identity motives and social identity.