Priority health-risk behaviors contribute to the leading causes of morbidity and mortality among youth and adults. Population-based data on these behaviors at the national, state, and local levels can help monitor the effectiveness of public health interventions designed to protect and promote the health of youth nationwide. September 2014-December 2015. The Youth Risk Behavior Surveillance System (YRBSS) monitors six categories of priority health behaviors among youth and young adults: 1) behaviors that contribute to unintentional injuries and violence; 2) tobacco use; 3) alcohol and other drug use; 4) sexual behaviors related to unintended pregnancy and sexually transmitted infections (STIs), including human immunodeficiency virus (HIV) infection; 5) unhealthy dietary behaviors; and 6) physical inactivity. In addition, YRBSS monitors the prevalence of obesity and asthma and other priority health behaviors. YRBSS includes a national school-based Youth Risk Behavior Survey (YRBS) conducted by CDC and state and large urban school district school-based YRBSs conducted by state and local education and health agencies. This report summarizes results for 118 health behaviors plus obesity, overweight, and asthma from the 2015 national survey, 37 state surveys, and 19 large urban school district surveys conducted among students in grades 9-12. Results from the 2015 national YRBS indicated that many high school students are engaged in priority health-risk behaviors associated with the leading causes of death among persons aged 10-24 years in the United States. During the 30 days before the survey, 41.5% of high school students nationwide among the 61.3% who drove a car or other vehicle during the 30 days before the survey had texted or e-mailed while driving, 32.8% had drunk alcohol, and 21.7% had used marijuana. During the 12 months before the survey, 15.5% had been electronically bullied, 20.2% had been bullied on school property, and 8.6% had attempted suicide. Many high school students are engaged in sexual risk behaviors that relate to unintended pregnancies and STIs, including HIV infection. Nationwide, 41.2% of students had ever had sexual intercourse, 30.1% had had sexual intercourse during the 3 months before the survey (i.e., currently sexually active), and 11.5% had had sexual intercourse with four or more persons during their life. Among currently sexually active students, 56.9% had used a condom during their last sexual intercourse. Results from the 2015 national YRBS also indicated many high school students are engaged in behaviors associated with chronic diseases, such as cardiovascular disease, cancer, and diabetes. During the 30 days before the survey, 10.8% of high school students had smoked cigarettes and 7.3% had used smokeless tobacco. During the 7 days before the survey, 5.2% of high school students had not eaten fruit or drunk 100% fruit juices and 6.7% had not eaten vegetables. More than one third (41.7%) had played video or computer games or used a computer for something that was not school work for 3 or more hours per day on an average school day and 14.3% had not participated in at least 60 minutes of any kind of physical activity that increased their heart rate and made them breathe hard on at least 1 day during the 7 days before the survey. Further, 13.9% had obesity and 16.0% were overweight. Many high school students engage in behaviors that place them at risk for the leading causes of morbidity and mortality. The prevalence of most health behaviors varies by sex, race/ethnicity, and grade and across states and large urban school districts. Long-term temporal changes also have occurred. Since the earliest year of data collection, the prevalence of most health-risk behaviors has decreased (e.g., riding with a driver who had been drinking alcohol, physical fighting, current cigarette use, current alcohol use, and current sexual activity), but the prevalence of other behaviors and health outcomes has not changed (e.g., suicide attempts treated by a doctor or nurse, smokeless tobacco use, having ever used marijuana, and attending physical education classes) or has increased (e.g., having not gone to school because of safety concerns, obesity, overweight, not eating vegetables, and not drinking milk). Monitoring emerging risk behaviors (e.g., texting and driving, bullying, and electronic vapor product use) is important to understand how they might vary over time. YRBSS data are used widely to compare the prevalence of health behaviors among subpopulations of students; assess trends in health behaviors over time; monitor progress toward achieving 21 national health objectives for Healthy People 2020 and one of the 26 leading health indicators; provide comparable state and large urban school district data; and help develop and evaluate school and community policies, programs, and practices designed to decrease health-risk behaviors and improve health outcomes among youth.