Although the emergence of sexual expression during adolescence and early adulthood is nearly universal, little is known about broad patterns of initiation. This dissertation identifies patterns of first oral-genital, anal, and vaginal sex and describes their implications for reproductive health and sexual risk taking in adulthood using data from the National Longitudinal Study of Adolescent Health (Add Health). In the first paper, I use latent class analysis to classify 12,194 respondents from Wave I and Wave IV of Add Health on the basis of variety, timing, spacing, and sequencing of oral-genital, anal, and vaginal sex. Approximately one half of respondents followed a pattern characterized predominately by initiation of vaginal sex first, average age of initiation of approximately 16 years, and spacing of one year or more between initiation of the first and second behaviors. Almost one third initiated sexual activity slightly later but reported first experiences of oral-genital and vaginal sex within the same year. Classes characterized by postponement of sexual activity, initiation of only one type of behavior, or adolescent initiation of anal sex were substantially less common. Compared to White respondents, Black respondents were more likely to appear in classes characterized by initiation of vaginal sex first. Respondents from lower socioeconomic backgrounds were more likely to be in classes distinguished by early/atypical patterns of initiation. The second paper describes associations between class membership and sexually transmitted infections, partner counts, concurrent sexual partnerships, and giving/receiving money for sex, with extensive controls for sociodemographics, behavioral, and psychosocial characteristics. I find that, with the exception of postponement of sexual activity until young adulthood, associations between adolescent sexual patterns and reproductive health/sexual risk taking are complex. While greater sexual variety is generally associated with poorer reproductive and sexual health outcomes, these relationships are not entirely consistent across outcomes and across classes. Greater attention to the relational and interpersonal contexts of early sexual patterns may help elucidate their implications for subsequent wellbeing. More broadly, this dissertation highlights the importance of considering the multiple interactive factors that contribute to processes of normative sexuality development.