"Priority health-risk behaviors" are defined as "interrelated and preventable behaviors that contribute to the leading causes of morbidity and mortality among youths and adults." The six categories of priority health-risk behaviors monitored by the YRBSS are behaviors that contribute to unintentional injuries and violence; sexual behaviors that contribute to human immunodeficiency virus (HIV) infection, other sexually transmitted diseases, and unintended pregnancy; tobacco use; alcohol and other drug use; unhealthy dietary behaviors; and physical inactivity. The latter categories include the prevalence of obesity and asthma among this population. The YRBSS obtains data on these behaviors from multiple sources, including a national school-based survey conducted by the Centers for Disease Control and Prevention (CDC), along with school-based State, territorial, tribal, and large urban school district surveys conducted by education and health agencies. These surveys have been conducted biennially since 1991. They involve representative samples of students in grades 9-12. The CDC first published the YRBSS methodology in 2004 (MMWR 2004:53, No RR-121). The current report on the YRBSS methodology includes improvements made since the 2004 report. These include increases in the YRBSS's coverage and expanded CDC technical assistance for entities that collect data used by the YRBSS. This report updates questionnaire content; operational procedures; sampling, weighting, and response rate; data-collection protocols; data-processing procedures; reports and publications; and data quality. Also included in this report are the results of methods studies that have systematically examined how various survey procedures influence prevalence estimates. There will continue to be ongoing revisions of the questionnaire, new population additions, and the development of innovative data-collection methods. 43 references, 2 exhibits, and 3 tables