We integrated findings from 35 recent, longitudinal studies of the onset of heterosexual intercourse. Correlates of adolescent sexual intercourse onset, whether in early (before age 16) or middle (ages 16-18) adolescence, included living with other than two biological parents, being less monitored by parents, having more advanced physical maturity and more involvement in dating behavior, and having more permissive attitudes toward sex. When studies were organized by age of participants, the onset of intercourse was more strongly associated with alcohol use, delinquency, school problems and (for girls) depressive symptoms in Early studies (sexual intercourse by age 15 or before only) than was found in studies classified as Middle (assessment of sexual intercourse up to age 18) or Late (knowledge of those who delayed until after age 18). Although more research is needed, additional factors were associated with delaying first sexual intercourse until after age 18, including religious attitudes and anxiety, with some factors more relevant for girls and some more applicable to boys. In total, the evidence suggests there are many similarities, but also some important differences, in the correlates associated with early versus middle versus later onset of sexual intercourse. This seems to signify more than one pathway (set of distal and proximal correlates) associated with sexual behavior during adolescence that should be tested in future research. Throughout the review, we highlight differences in the correlates of girls' versus boys' sexual intercourse and how race/ethnicity moderates associations. These gender and racial/ethnic differences were found largely in analyses of family processes, school and religion, and parent education. We end by summarizing several priority areas for future research.