Migration is transforming our world: by the end of this decade, most developing countries will have more people living in cities than in rural areas. Adolescent girls in developing countries are an overlooked population, but one that is migrating to urban areas in ever greater numbers. Girls are on the move because they lack opportunity in their rural hometowns, and want to work, to learn, and to gain skills and resources. They also move to escape hardship: poverty, war, or early marriage. While migration can carry risks, for many girls moving can unlock opportunity, autonomy, and the chance for prosperity.