In the United States, approximately three fourths of all deaths among persons aged 10--24 years result from only four causes: motor-vehicle crashes, other unintentional injuries, homicide, and suicide. Results from the 1999 national Youth Risk Behavior Survey demonstrate that numerous high school students engage in behaviors that increase their likelihood of death from these four causes --- 16.4% had rarely or never worn a seat belt; during the 30 days preceding the survey, 33.1% had ridden with a driver who had been drinking alcohol; 17.3% had carried a weapon during the 30 days preceding the survey; 50.0% had drunk alcohol during the 30 days preceding the survey; 26.7% had used marijuana during the 30 days preceding the survey; and 7.8% had attempted suicide during the 12 months preceding the survey. Substantial morbidity and social problems among young persons also result from unintended pregnancies and STDs, including HIV infection. In 1999, nationwide, 49.9% of high school students had ever had sexual intercourse; 42.0% of sexually active students had not used a condom at last sexual intercourse; and 1.8% had ever injected an illegal drug.