The YRBSS is an epidemiologic surveillance system that was recently established by the CDC to monitor the prevalence of youth behaviors that most influence health. The introduction to this collection of articles describes why the YRBSS was established, what methods were used to delineate the behaviors monitored through the system, and how the survey instrument will be used to periodically produce national, State, and local data on these behaviors. Each article focuses on health-related behavior or behaviors documented through the 1990-1991 YRBSS. The behaviors addressed in terms of their prevalence among high school students are participation in school physical education; tobacco use; attempted suicide; current tobacco, alcohol, marijuana, and cocaine use; weapon-carrying; body-weight perceptions and selected weight management goals and practices; alcohol and other drug use; sexual behavior; vigorous physical activity; physical fighting; safety-belt and helmet use; selected behaviors that increase the risk for HIV infection; selected tobacco-use behaviors and dietary patterns; behaviors related to unintentional and intentional injuries; HIV instruction and selected HIV-risk behaviors; and selected behaviors that increase the risk for HIV infection, other sexually transmitted diseases, and unintended pregnancy. Data tables are included in each report.