This study explicates mechanisms of media campaign effectiveness in the context of children's physical activity. The authors' model expands the theory of planned behavior by integrating injunctive and descriptive norms into its normative mechanism. Analysis of a 3-wave nationally representative evaluation survey among 1,623 tweens indicates that campaign exposure is significantly related, but only indirectly, to both physical activity intention and physical activity behavior. Instead, campaign exposure seems more strongly related to perceived behavioral control and attitudes toward physical activity. By contrast, perceived behavioral control and descriptive norms are strongly related to behavioral intention. The findings suggest that integrating normative mechanisms with the theory of planned behavior can improve efforts to predict and explain a health behavior.