African American adolescents are a high risk for HIV and other sexually transmitted diseases (STDs). Prior HIV prevention programs have had limited success with this population. We propose testing an innovative method for constructing intervention groups and delivering an intervention that synthesizes the most effective components of prior hIV programs for adolescents. This work will be conducted i two neighborhoods with large populations of adolescents at high risk of hIV/STDs and will include in-and-out-of-school youth. The group composition experiment will construct target groups based on social cliques derived from neighborhood crowd networks. We will compare the effects of the intervention in the clique-based condition to the effects of the same intervention for target groups developed through the standard method of pooling participants without regard to crowd and clique membership. We will: . Evaluate the cognitive, behavioral, and biological outcomes of an intervention delivered to groups that are targeted based on social cliques and compare the results with those in groups reached by the standard "all comers" approach. . Examine hypothesized mediators of the intervention effects on the outcomes associated with the four stages of the AIDS Risk Reduction Model (ARRM). . Examine the cost effectiveness of the clique-based intervention compared to the standard approach in terms of the number of chlamydia and gonorrhea infections averted, and by mathematical modeling, the cost per HIV infection averted. We will develop an intervention to be delivered in small groups to 430 adolescents aged 15-19 in each participating community (total N=860). The three session intervention is based on the ARRM. An individualized booster session will be conducted at 4 months post-intervention. Attitudinal and behavioral assessments will be conducted at two points pre- intervention, immediately post-intervention, and at 4 and 12 months post intervention. Urine samples for analysis of gonorrhea and chlamydia will be obtained at the second baseline and at 12 month follow-up. These outcomes will allow us to assess the effects of interventions delivered to groups of adolescents selected on the basis of their social factors.