Two studies on the Theory of Planned Behavior (TPB) and exercise behavior are reported that consider the mediating and moderating effects of planning on intention-behavior relationships. Undergraduate students (N = 125 and N = 102) completed questionnaires assessing TPB constructs, planning, and past exercise behavior. The TPB was highly predictive of exercise intentions (R² = .37 and .62) and future behavior (R² = .43 and .49) assessed at 2 weeks (Study 1) and 1 week (Study 2) follow-up. Planning was found to mediate the impact of intention on future behavior (Study 2) and to moderate the intention/behavior relationship (both studies). The results are discussed in relation to recent models of health behavior that focus on the volitional (i.e., postdecisional) phase of health behavior.