<p>This thesis tested and extended the peer socialization hypothesis, a psychosocial model that identifies the interpersonal and contextual conditions in which pubertal development is linked to delinquent behavior such as alcohol use. Although there is support for the hypothesis in early empirical work, particularly for female adolescents, more recent work has shown mixed results. Furthermore, there are gaps within the peer socialization hypothesis: a) the theory does not discuss if the mediation process pertains to male adolescents, b) it does not address the role of pubertal tempo, and c) it implies, but does not explicitly model, a role for tempo with regards to including other-sex friendships into the friend group. Using the Internet Surveys About You (iSAY) study (<em>n</em>=1020), the peer socialization hypothesis and proposed extensions were tested separately by sex. Female adolescents initiated inclusion of other-sex friends into the friend group earlier than male adolescents. However, there were no observed sex differences in the tempo of other-sex friendships. Female adolescents’ pubertal development was not related to their other-sex friendship development nor was pubertal development and other-sex friendship development related to their alcohol use. Male adolescents’ pubertal timing was associated with the timing of other-sex friendship uptake in that later maturing adolescents displayed earlier initiation of other-sex friendships. Male adolescents’ other-sex friendship development was not related to their alcohol use. Implications of this work are that other-sex friendships may no longer be a context of increased risk for alcohol use for early maturing adolescents and interventions targeting this social context may not be as effective. </p>