In this paper we contrast the two methods that are most frequently used by developmental psychologists to identify children's or adolescents' affiliative peer groups. Following sociometric traditions, the first method requests participants to nominate their own preferred peer partners. In the second method, socio-cognitive mapping, children or adolescents are seen as participant observers who report on their own groups as well as on other groups of peers they frequently observe in their setting. First, we trace the history of the methods. Then, we present their theoretical assumptions, methodological procedures, and analytic strategies. For each method, we provide an example, using software tools that | were designed for the specific kinds of data. Finally, we recommend specific criteria that, may guide researchers' decisions for adopting one of the alternatives.