The "Morbidity and Mortality Weekly Report" ("MMWR") series of publications is published by the Office of Science, Centers for Disease Control and Prevention (CDC), U.S. Department of Health and Human Services. Articles included in this supplement are: (1) Overview and Methods for the Youth Risk Behavior Surveillance System--United States, 2021; (2) School Connectedness and Risk Behaviors and Experiences Among High School Students--Youth Risk Behavior Survey, United States, 2021; (3) Witnessing Community Violence, Gun Carrying, and Associations with Substance Use and Suicide Risk Among High School Students--Youth Risk Behavior Survey, United States, 2021; (4) Experiences of Unstable Housing Among High School Students--Youth Risk Behavior Survey, United States, 2021; (5) Parental Monitoring and Risk Behaviors and Experiences Among High School Students--Youth Risk Behavior Survey, United States, 2021; (6) Suicidal Thoughts and Behaviors Among High School Students--Youth Risk Behavior Survey, United States, 2021; (7) Role of the COVID-19 Pandemic on Sexual Behaviors and Receipt of Sexual and Reproductive Health Services Among U.S. High School Students--Youth Risk Behavior Survey, United States, 2019-2021; (8) Dating Violence, Sexual Violence, and Bullying Victimization Among High School Students--Youth Risk Behavior Survey, United States, 2021; (9) Dietary and Physical Activity Behaviors in 2021 and Changes from 2019 to 2021 Among High School Students--Youth Risk Behavior Survey, United States, 2021; (10) Alcohol and Other Substance Use Before and During the COVID-19 Pandemic Among High School Students--Youth Risk Behavior Survey, United States, 2021; and (11) Electronic Vapor Product Use Among High School Students--Youth Risk Behavior Survey, United States, 2021.