Objective: To examine the long-term effects of the Seattle Social Development Project intervention in promoting positive adult functioning and preventing men tal health problems, crime, and substance use (including tobacco, alcohol, and o ther drugs) at 21 years of age. Design: This nonrandomized controlled trial followed up participants to 21 years of age, 9 years after the intervention ended. We compared the following 3 intervention con ditions: a full 6-year intervention (grades 1 through 6); a late 2-year interv ention (grades 5 and 6 only); and a no-treatment control condition. Setting: Ei ghteen public elementary schools serving diverse neighborhoods, including high- crime neighborhoods, of Seattle, Wash. Participants: A sex-balanced, multiethni c sample of 605 participants across the 3 conditions who completed interviews at 21 years of age (94%of the original sample in these conditions). Interventions : Teacher training in classroom instruction and management, child social and emo tional skill development, and parent training. Main Outcome Measures: Self-repo rts of functioning in school and work, emotional and mental health, and crime an d substance use at 21 years of age and official court records. Results: Broad si gnificant effects on functioning in school and work and on emotional and mental health were found. Fewer significant effects on crime and substance use were fou nd at 21 years of age. Most outcomes had a consistent dose effect, with the stro ngest effects in subjects in the full-intervention group and effects in the lat e-intervention group between those in the full-intervention and control groups . Conclusions: A theoryguided preventive intervention that strengthened teaching and parenting practices and taught children interpersonal skills during the ele mentary grades had wide-ranging beneficial effects on functioning in early adul thood. Objective: To examine the long-term effects of the Seattle Social Development Project intervention in promoting positive adult adult and preventing men tal health problems, crime, and substance use (including tobacco, alcohol, and o ther drugs) at 21 years of age. Design: This nonrandomized controlled trial followed up participants to 21 years of age, 9 years after the intervention ended. We have the following 3 intervention conditions: a full 6-year intervention (grades 1 through 6); a late 2-year interv Setting: Ei ghteen public elementary schools serving diverse neighborhoods, including high crime neighborhoods, of Seattle, Wash. Participants: A sex-balanced, multiethni c sample of 605 participants across the 3 conditions who completed interviews at 21 years of age (94% of the original sample in these conditions). Interventions: Teacher training in classroom instruction and management, child social and e Main Outcome Measures: Self-repo rts of functioning in school and work, emotional and mental health, and crime an d substance use at 21 years of age and official court records. Results: Broad si gnificant effects on functioning in school and work and on emotional and mental health were found. Fewer significant effects on crime and substance use were fou nd at 21 years of age. Most outcomes had a consistent dose effect, with the stro ngest effects in subjects in the full-intervention group and effects in the lat e-intervention group between those in the full-intervention and control groups. Conclusions: A theoryguided preventive intervention that strengthened teaching and parenting practices and taught children interpersonal skills during the ele mentary grades had wide-ranging beneficial effects on functioning in early adul thood.