Individuals who lack perceived self-efficacy may benefit less from planning interventions because they fail to apply their plans when encountering situations that appear challenging. Thus, self-efficacy may operate as a moderator in relation to the effects of planning interventions on behaviour. This article reports from two experimental studies using cluster randomisation. Study 1 included 58 patients with diabetes who received a face-to-face action planning intervention to improve their physical activity levels. Study 2 targeted the roles of planning and self-efficacy for the maintenance of regular running and for relapse prevention in 187 active individuals over a 2-year period. The email-delivered intervention prompted specific goal setting and facilitated planning to mobilise social support to secure relapse prevention. Results of the two studies were consistent with a moderating effect of self-efficacy in relation to the planning-behaviour relationship. Individuals with high self-efficacy benefited from planning interventions. If a person lacks self-efficacy, planning may not effectively promote physical activity.