Priority health-risk behaviors (i.e., interrelated and preventable behaviors that contribute to the leading causes of morbidity and mortality among youths and adults) often are established during childhood and adolescence and extend into adulthood. The Youth Risk Behavior Surveillance System (YRBSS), established in 1991, monitors six categories of priority health-risk behaviors among youths and young adults: 1) behaviors that contribute to unintentional injuries and violence; 2) sexual behaviors that contribute to human immunodeficiency virus (HIV) infection, other sexually transmitted diseases, and unintended pregnancy; 3) tobacco use; 4) alcohol and other drug use; 5) unhealthy dietary behaviors; and 6) physical inactivity. In addition, YRBSS monitors the prevalence of obesity and asthma among this population. YRBSS data are obtained from multiple sources including a national school-based survey conducted by CDC as well as schoolbased state, territorial, tribal, and large urban school district surveys conducted by education and health agencies. These surveys have been conducted biennially since 1991 and include representative samples of students in grades 9-12. In 2004, a description of the YRBSS methodology was published (CDC. Methodology of the Youth Risk Behavior Surveillance System. MMWR 2004;53 [No RR-12]). Since 2004, improvements have been made to YRBSS, including increases in coverage and expanded technical assistance.This report describes these changes and updates earlier descriptions of the system, including questionnaire content; operational procedures; sampling, weighting, and response rates; data-collection protocols; data-processing procedures; reports and publications; and data quality. This report also includes results of methods studies that systematically examined how different survey procedures affect prevalence estimates. YRBSS continues to evolve to meet the needs of CDC and other data users through the ongoing revision of the questionnaire, the addition of new populations, and the development of innovative methods for data collection.