This article describes findings of the third Project STARS sub-project, which investigated the role of the Internet in adolescent sexual development. The empirical studies that are conducted within this sub-project aimed to offer more insight into (1) the prevalence and development of different sex-related online behaviors in adolescence; (2) their longitudinal associations with sexual cognitions, behaviors, and emotions; and (3) how sex-related Internet use is embedded in personal and social contexts. The findings suggest that for most adolescents, engagement in most sex-related online behaviors is much less excessive than is often assumed or feared. Nonetheless, the longitudinal and qualitative data confirm theoretical assumptions by showing that sex-related online behaviors may, through a variety of mechanisms, shape adolescents’ sexual developmental processes. Furthermore, the findings highlight that sexrelated Internet use does not occur in isolation, but is intertwined with various intra- and interpersonal processes. For example, sex-related behaviors were found to predict increases in adolescents’ perceptions that sexual behavior is common and accepted among peers, which in turn predicted increases in their own experience with sexual behavior. Moreover, the various studies showed that many congruencies exist between adolescents’ own dispositions, developmental interests, and social setting on the one hand and their patterns of sex-related Internet use on the other hand. The findings of this sub-project contribute to theory and knowledge about the role of the Internet in adolescent sexual development. They also contain relevant implications for parents, schools and intervention developers who strive to promote a healthy and positive sexual development.