The initiation of romantic and sexual relationships during adolescence is a normative and integral part of adolescent identity development. In the United States, the majority of young people have romantic relationships and initiate sexual activity during their high school years, and this trend has continued for decades. In this chapter, we review recent trends in adolescent sexual attractions, identities, and behaviors, including debut, practices, and partnerships, and discuss current trends in contraceptive and protective behaviors and outcomes. In addition, we attend to the cultural context within which young people in the United States initiate sexual relationships, and its meaning for adolescent sexual desire, identity, and well-being. We summarize important data and methodological issues in research on adolescent sexuality, offering suggestions for future research areas and data needs. Finally, we place adolescent sexuality in the United States in a larger context, ending with an international comparison of adolescent sexuality.