Priority health-risk behaviors, which are behaviors that contribute to the leading causes of morbidity and mortality among youth and adults, often are established during childhood and adolescence, extend into adulthood, are interrelated, and are preventable. January--December 2007. The Youth Risk Behavior Surveillance System (YRBSS) monitors six categories of priority health-risk behaviors among youth and young adults, including behaviors that contribute to unintentional injuries and violence; tobacco use; alcohol and other drug use; sexual behaviors that contribute to unintended pregnancy and sexually transmitted diseases (STDs), including human immunodeficiency virus (HIV) infection; unhealthy dietary behaviors; and physical inactivity. In addition, YRBSS monitors the prevalence of obesity and asthma. YRBSS includes a national school-based survey conducted by CDC and state and local school-based surveys conducted by state and local education and health agencies. This report summarizes results from the national survey, 39 state surveys, and 22 local surveys conducted among students in grades 9--12 during 2007. In the United States, 72% of all deaths among persons aged 10--24 years result from four causes: motor-vehicle crashes, other unintentional injuries, homicide, and suicide. Results from the 2007 national Youth Risk Behavior Survey (YRBS) indicated that many high school students engaged in behaviors that increased their likelihood of death from these four causes. Among high school students nationwide during 2007, 11.1% had never or rarely worn a seat belt when riding in a car driven by someone else. During the 30 days before the survey, 29.1% of high school students had ridden in a car or other vehicle driven by someone who had been drinking alcohol, 18.0% had carried a weapon, and 5.5% had not gone to school because they felt they would be unsafe at school or on their way to or from school. During the 12 months before the survey, 6.9% of high school students had attempted suicide. In addition, 75.0% of high school students had ever drunk alcohol, and 4.4% had ever used methamphetamines. Substantial morbidity and social problems among youth also result from unintended pregnancies and STDs, including HIV infection. Results from the 2007 survey indicated that 47.8% of students had ever had sexual intercourse, 35.0% of high school students were currently sexually active, and 38.5% of currently sexually active high school students had not used a condom during last sexual intercourse. Among U.S. adults aged >or=25 years, 59% of all deaths result from two causes: cardiovascular disease and cancer. Results from the 2007 national YRBS indicated that risk behaviors associated with these two causes of death were present during adolescence. Among high school students nationwide during 2007, 20.0% had smoked cigarettes during the 30 days before the survey, 35.4% had watched television 3 or more hours per day on an average school day, and 13.0% were obese. During the 7 days before the survey, 78.6% of high school students had not eaten fruits and vegetables five or more times per day, 33.8% had drunk soda or pop at least one time per day, and 65.3% had not met recommended levels of physical activity. Since 1991, the prevalence of many health-risk behaviors among high school students nationwide has decreased. However, many high school students continue to engage in behaviors that place them at risk for the leading causes of mortality and morbidity. The prevalence of most risk behaviors does not vary substantially among cities and states. YRBS data are used to measure progress toward achieving 15 national health objectives for Healthy People 2010 and three of the 10 leading health indicators, to assess trends in priority health-risk behaviors among high school students, and to evaluate the impact of broad school and community interventions at the national, state, and local levels. More effective school health programs and other policy and programmatic interventions are needed to reduce risk and improve health outcomes among youth.