The negative outcomes of early childbearing and sexually transmitted infections (STIs), including HIV/AIDS, threaten the health of adolescents more than any other age group. Ensuring the sexual and reproductive health of the more than 1.5 billion young people aged 10 to 25 around the world is central to global health. Country-level indicators show dramatic variations in sexual risk. Percentages of those who engage in sexual intercourse range from less than 1% of females in Pakistan to 54% of males in Cuba. Divergent rates of early pregnancy and STIs between countries and regions parallel variations in sexual behaviors, including age of sexual debut; number of partners; and use of contraception, including condoms. To understand these variations, many factors affecting the sexual and reproductive health of young people around the world such as age of marriage, norms and expectations around sexual behavior, gender inequities, and educational and economic opportunities must be considered.