CONTEXT: The promotion of optimism has been widely advocated for children and adolescents, but epidemiologic data to support this approach are scant. METHODS: This was a 3-wave longitudinal study of health and social development in younger adolescents from 3 Australian states. The 5634 student participants, initially aged 12 to 14 years, were assessed for optimistic thinking style, emotional problems, substance use, and antisocial behaviors. RESULTS: Cross-sectional associations between optimism and each of the study outcomes were strongly protective but tended to differ according to gender in extent. In prospective analyses of the onset of new cases of each study outcome, protective associations were weaker. Those in the highest optimism quartile had risks for depressive symptoms that were reduced by almost half (odds ratio: 0.54 [95% confidence interval: 0.42- 0.70]) compared with those in the lowest category. No effect was seen in prevention of anxiety symptoms after adjustment for other aspects of psychological style. In predicting the onset of heavy substance use and antisocial behavior, high optimism had modest protective effects. CONCLUSIONS: Optimistic thinking style is somewhat protective against adolescent health risks; the clearest effects are seen against depressive symptoms. Promoting optimism along with other aspects of psychological and emotional style has a role in mental health promotion that is likely to be enhanced if an intervention also addresses risk and protective factors in an adolescent's social context.