This article presents the findings from the first part of the Project STARS study on adolescent romantic and sexual development using a biopsychosocial perspective. With five empirical studies, the role of individual factors (personality, pubertal development) and social context (peers, sexualized media) for the romantic and sexual development are examined. A meta-analysis showed that an early and more advanced pubertal development is associated with more (risky) sexual experiences. Further, a longitudinal study showed that pubertal development is also related to changes in social status, and that this partly explains why ‘early’ and ‘rapidly’ developing adolescents begin intimate relationships at younger ages. Personality was also found to be an important factor for adolescent sexual development: Adolescents with an undercontrolling personality type (versus resilient and overcontrolling) were found to engage in more (risky) sexual behaviors. Further, personality (low emotional stability) was related to a preference for friends with a similar level of sexual intention. Finally, with a longitudinal study sexualized media was shown to play a role in the development of permissive sexual attitudes— especially for youth who perceived these media to be realistic. The findings of this dissertation show that individual differences in adolescent romantic and sexual development can be explained by differences in personality and pubertal development, in interaction with social context. Despite the normativity of sexual development during adolescence, the results also indicate which youth are more vulnerable to risky and negative experiences.