Many individuals are motivated to improve their physical activity levels but often fail to act on their good intention. This study examines the roles of planning and self-efficacy in the prediction of physical activity. A total of 290 participants (77% women, mean age = 41.9 years) were surveyed three times. Intentions, planning, and physical activity were specified as a mediator chain. Results reveal that intentions were partly translated into physical activity by planning. Self-efficacy moderated this mediation, reflected by a planning × self-efficacy interaction ( p < .05) on physical activity accounting for 16% of the variance in behavior. If a person is self-efficacious, planning seems more likely to be translated into physical activity.