Adolescent girls are on the move. Throughout the developing world, natural disasters, political emergencies, child marriage, and violence force a migration journey many girls do not choose. But for most girls, migration is a choice made in a quest for social and economic advancement. Migration offers adolescent girls the possibility and promise of opportunity. Absent from their parents and the limits of their natal homes, migrant girls can seize new opportunities and fully enjoy their human rights as they enter adulthood. The ability to navigate a number of critical milestones throughout her journey shapes a migrant girl's success or failure. An unsuccessful transition can have significant consequences. It is those consequences that are portrayed in popular culture: the trafficked Eastern European girl who is sexually exploited; the young Chinese woman hunched over a sewing machine in filthy surroundings; the frightened domestic worker in Central America. These disadvantages are real and deserve attention, but they do not tell the whole story. This report brings a fresh image of girls' migration: the uniformed secondary school student in a boarding school in Zimbabwe; the apprentice acquiring tailoring skills in Ghana; and the factory worker in Bangladesh abandoning her traditional role to negotiate higher wages with an employer. Should a migrant girl successfully navigate her journey, the ability to unlock her potential expands enormously. When she thrives in her new home, a successful migrant girl can be a powerful agent of social and economic change, transforming the prospects of her family and community. She has a unique potential, known as the girl effect, to end poverty for herself and for others. With a primary focus on internal migration, "Girls on the Move" 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. Building from the evidence, the report 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.