Screening, combined with appropriate intervention and referral, has the potential to significantly reduce alcohol and marijuana problems in youth. However, most screens perform poorly or have not been validated in youth, or are too long and unwieldy to be used in busy clinical settings. There is a great deal of interest in consumption-based youth screens that are developmentally graded and very brief, and that can detect both current substance problems as well as risk for future problems. A primary example is NIAAA's new Alcohol Screening Guide for Children and Adolescents, which uses questions about the frequency of alcohol use, and about friends' drinking, to identify youth that have or are at risk fr alcohol problems. Age-graded cut- points for the alcohol frequency question were derived from analyses of respondents age 12-18 in a representative, cross-sectional national survey using DSM-IV-based outcomes. However, much more work is needed to study the concurrent and predictive validity of such consumption- based screening. We propose to conduct secondary data analyses in five studies (three of which have long-term longitudinal data) to characterize the validity of consumption-based alcohol and marijuana screening in youth aged 8 to 20. We will study the performance of alcohol and marijuana consumption screening questions in children, adolescents and young adults, using epidemiologic samples, and studies of youth in primary care, in addictions treatment, and entering college. We will examine whether consumption-based screening is invariant by race- ethnicity. We will study screening cut-points in the context of proposed DSM-5 criteria for Alcohol and Cannabis Use Disorders, but will emphasize milder forms of substance problems that are more common in youth. We will disseminate our results by making new screening products easily accessed via a web site. Our results will inform NIAAA's ongoing efforts to develop an on-line CME course, and an interactive mobile web screening app, based on the Guide. This project is highly significant because it will greatly increase scientific knowledge supporting use of brief consumption-based alcohol and marijuana screening in youth.