Adolescent girls in developing countries are migrating to urban areas in ever greater numbers. While migration can be risky, for the majority of girls, migration can increase opportunities and economic stability and provide them with the autonomy to make decisions about their lives. This report is the first of its kind to examine the social and economic determinants of internal migration for adolescent girls in developing countries, and to identify the links between migration, risk, and opportunity. With a primary focus on internal migration, the report presents evidence not only about how and why a girl moves, but ultimately about what factors enable girls to take advantage of new opportunities in employment, education, and health. It finally recommends policies and programs for migrant adolescent girls that take account of their diversity, ensure protection, and cultivate connections, assets, and capital—the main ingredients for migrant girls’ success, according to the authors.