CDC developed the Youth Risk Behavior Surveillance System (YRBSS) to monitor six categories of priority health-risk behaviors among youth--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, including human immunodeficiency virus (HIV) infection; unhealthy dietary behaviors; and physical inactivity--plus overweight. These risk behaviors contribute markedly to the leading causes of death, disability, and social problems among youth and adults in the United States. YRBSS includes a national school-based survey conducted by CDC as well as state, territorial, and local school-based surveys conducted by education and health agencies. In these surveys, conducted biennially since 1991, representative samples of students in grades 9--12 are drawn. In 2003, a total of 15,214 students completed the national survey, and 32 states and 20 school districts also obtained data representative of their jurisdiction. Although multiple publications have described certain methodologic features of YRBSS, no report has included a comprehensive description of the system and its methodology. This report describes the background and rationale for YRBSS and includes a detailed description of the methodologic features of the system, including its questionnaire; operational procedures; sampling, weighting, and response rates; data-collection protocols; data-processing procedures; reports and publications; and data quality. YRBSS is evolving to meet the needs of CDC and other users of the data.