Planning is assumed to translate intentions into health behaviors. However, this may fail due to a lack of perceived self-efficacy. People do not tackle challenging tasks if they harbor self-doubts, even if they have made a good action plan. The present two descriptive longitudinal studies are designed to examine the putative moderating role of self-efficacy in the planning-behavior relationship. In Study I (N = 534 Chinese adolescents), intentions were assessed at baseline, whereas planning, self-efficacy, and physical activity were measured four weeks later. In Study II, 620 Polish adolescents filled out questionnaires assessing physical activity, intentions, planning, and selfefficacy with a 10-week follow-up assessment of physical activity. A moderated mediation model was examined. Planning was specified as a mediator between intentions and behavior, whereas self-efficacy was specified as a moderator of the planningbehavior relationship. Results confirm that levels of self-efficacy moderate the mediation process. The strength of the mediated effect (intention via planning on behavior) increased along with levels of self-efficacy. These results remained valid after accounting for baseline physical activity. For planning to mediate the intention-behavior relation, adolescents must have sufficiently high levels of self-efficacy. Otherwise, planning might be in vain. Implications for theory advancement and intervention development are discussed.