In the past quarter century, research on adolescence has expanded from a near exclusive focus on intraindividual processes to a concern with individuals in an interpersonal context. Today, studies of the impact of relationships within families, with peers, and with romantic partners account for a large proportion of research in the field. This article outlines three features of this transformation: an increasing focus on the nature of, changes in, and the developmental impact of adolescents' relationships with significant others; the expansion and diversification of networks of significant others during adolescence; and the recognition of significant interrelations among these relationships. Contemporary studies require research designs that encompass multiple significant relationships and that assess a broad range of relationship properties.