We reviewed the existing empirical literature to assess cognitive and situational factors that may affect the validity of adolescents' self-reports of alcohol and other drug use, tobacco use, behaviors related to unintentional injuries and violence, dietary behaviors, physical activity, and sexual behavior. Specifically, we searched for peer-reviewed journal articles published in 1980 or later that examined the factors affecting self-report of the six categories of behavior listed above. We also searched for studies describing objective measures for each behavior. Self-reports of each of six types of health-risk behaviors are affected by both cognitive and situational factors. These factors, however, do not threaten the validity of self-reports of each type of behavior equally. The importance of assessing health-risk behaviors as part of research activities involving adolescents necessitates the use of self-report measures. Researchers should familiarize themselves with the threats to validity inherent in this type of assessment and design research that minimizes these threats as much as possible.