In the current studies, the authors examined how peers influence friendship choices through individuals' perceptions of similarity between their own and others' Big Five traits. Self-reported and peer-reported data were gathered from 3 independent samples using longitudinal round-robin designs. Peers' ratings of how similar 2 persons appeared in extraversion and agreeableness predicted friendship formation likelihood between these 2 persons in all samples. This association was mediated by perceived similarity. Furthermore, another mediation effect was found for similarity in interaction style: Persons who were viewed by peers as having similar extraversion and agreeableness levels became more similar in interaction styles. Thus, the current studies indicate that extraversion and agreeableness influence the emergence of social relationships through intrapersonal perceptions of similarity and interpersonal social interactions. We encourage researchers to look at specific similarity effects that influence interpersonal and intrapersonal processes to understand how relationships are formed.