This article examines adolescent sexuality within the context of a life course developmental model and considers the causal interrelationships of sexual behavior with dating, courtship, and planning for marriage. Data from a sample of 18-year-old men and women document the importance of dating and courtship development on the initiation of intimate heterosexual relations. Young people who begin to date at an early age also tend to develop steady relationships relatively early and continue to date more frequently. Both the timing of the initiation of dating and the development of steady dating relationships have substantial implications for the development of sexual relations. Young women and men who begin dating early and who develop steady relations early are more likely to be sexually experienced, to have had sexual relations with more partners, to be more sexually active during their late teenage years, and to have more permissive attitudes concerning premarital sex. Among those who experience sexual intercourse, those who had their first experience at a relatively young age had the most partners and had intercourse more frequently when they were 18.