In the United States, many adolescent girls experience sexual harassment before they leave high school, and between 20% and 25% of college women are survivors of sexual assault. Despite the many negative consequences associated with these experiences, perpetrating sexual harassment and assault is often viewed as normative. Using Bronfenbrenner’s bioecological theoretical framework, we propose a bioecological theory of the perpetration and tolerance of sexual harassment of girls. We propose children’s proximal and distal contexts contribute to the endorsement of sexualized gender stereotypes, which in turn impacts high rates of both perpetration and acceptance of sexual harassment. We discuss the ways that three important microsystems—parents, peers, and schools—contribute to this acceptance. We also propose that key components of media within the exosystem work to further normalize sexual harassment of girls and women. These contexts inform children’s development, creating a culture that is permissive of sexual harassment, resulting in high rates of sexual harassment and assault in adolescence and emerging adulthood. Implications of our proposed theory for policymakers, teachers, parents, and researchers are discussed.