Guided by the theory of planned behavior, this 2-week longitudinal study examined health behaviors in a sample of 279 adolescents. Social norms and perceived behavioral control (PBC) were tested as predictors of self-reported intentions and behaviors in 2 domains, eating and physical activity. Differentiating, as opposed to aggregating, parent and peer norms provided unique information. For PBC, the authors distinguished global causality beliefs from self-related agency beliefs and intraself (effort, ability) from extraself (parents, teachers) means. Intraself agency beliefs strongly predicted healthy intentions, whereas intraself causality beliefs had a negative influence. Patterns differed somewhat across behaviors and gender. Results highlight theoretical issues and provide potential targets for research on health promotion programs for youth.