Using longitudinal data from the National Survey of Children, we examined the impact of community socioeconomic status on four dimensions of adolescent and young adult premarital sexual activity—the timing of first intercourse, the frequency of intercourse, the number of different sex partners, and the likelihood of engaging in unprotected intercourse. We found significant positive effects of a multiitem index of community socioeconomic disadvantage on all but the timing of first premarital intercourse, net of controls for the socioeconomic and demographic status of adolescents and their families. None of the most commonly cited explanations for neighborhood effects on adolescent behavior can fully explain these associations. Only the attitudes and behaviors of peers account for even a small portion of the observed impact of community disadvantage on youth sexual behavior. Adolescents' acceptance of premarital childbearing, educational aspirations and attachment to school, and parental supervision, although frequently associated with youth sexual behavior, do little to mediate the impact of community disadvantage on sexual activity.