The need for the proposed study rests on the harmful consequences of early sexual debut alongside the pivotal increase in teen sexual behavior from 8th to 9th grade and the potential of family communication about sex to reduce this risky behavior. Few studies investigate teen-parent sexual communication during this transitional time or address the changing family demographics, which increasingly include extended family members. The proposed work will longitudinally explore continuity and change in teen-family sexual communication over the transition to high school, extend parent-teen dyadic studies of family sexual communication to encompass broader family networks, and explore the associations of such communication with teen sexual risk behavior. Findings from a large-scale, randomized control, longitudinal evaluation of a sex education program (N=1682 early adolescents) provide preliminary evidence that 6th. grade family sexual communication significantly predicts lower 7th. grade onset of sexual behavior, and identified high levels of reported extended-family involvement in teen sexual communication (69% in 8th. grade), with similar reports from a pilot interview study of 32 teens and parents. The proposed study will take a mixed method approach. (1) Secondary analysis from 8th and 9th grade surveys will examine how the content, quantity, and quality of teen-parent sexual communication predict change in teen sexual behavior, and will additionally explore extended-family sexual communication and its associations with teen sexual risk-taking. Quantitative findings will shape the content of interviews. (2) Primary interview data from 10th. grade students and their parents (also interviewed when teens were in 7th. grade) and interviews with extended-family members identified as important sources of teen sexual communication will explore the protective mechanisms for teen-family sexual communication. Analysis of 7th. and 10th. grade interview data will allow for investigation of continuity and change in teen-parent sexual communication. Analysis of extended-family interviews will cross-sectionally explore the content and process of teen-extended-family sexual communication. Content analysis will involve developing, coding, and interpreting of themes. The proposed study will produce recommendations for sex education programs based on changing adolescent and family roles over the transition to high school and involvement of extended family members in teen sexual communication. It will also direct physicians, nurses, counselors, and other health care providers on how to guide parents and extended-family members in talking with teens about sexual issues. Public Health Relevance: This mixed method study addresses the harmful consequences of early sexual debut alongside the pivotal increase in teen sexual behavior from 8th to 9th grade and the potential of family communication about sex to reduce this risky behavior. To address changing teen-parent relationships over the transition to high school and demographic shifts away from nuclear family structure, we will longitudinally investigate continuity and change in teen-parent sexual communication and explore the role of extended family in such conversations, assessing how they influence teens' sexual behavior. This work will produce recommendations for family intervention based on changing adolescent and family roles over the transition to high school and direct physicians, nurses, counselors, and other health care providers on how to identify and guide parents and extended family members in talking with teens about sexual issues.