Adolescence is a decisive age for girls around the world. What transpires during a girl’s teenage years shapes the direction of her life and that of her family. For many girls in developing countries, the mere onset of puberty that occurs during adolescence marks a time of heightened vulnerability—to leaving school, child marriage, early pregnancy, HIV, sexual exploitation, coercion and violence. Adolescent girls are less likely than older women to access sexual and reproductive health care, including modern contraception and skilled assistance during pregnancy and childbirth.