Nearly half of public high school students in New York City (NYC) and nationwide say they have had sex. About 1 in 3 are currently sexually active (had sex in the last 3 months). Encouraging adolescents to delay sex—or, if sexually active, to prevent pregnancy and sexually transmitted infections (STIs)—requires education, support, and services. Without access to birth control, sexual activity during adolescence poses health, social, and economic risks including pregnancy and STIs. Left untreated, curable but often asymptomatic STIs, such as chlamydia, can cause infertility and other serious health problems.