Objective: Although health cognitions are regarded as key determinants for health behavior change, they are rarely studied from a dynamic perspective, evaluating the impact of changes in cognitions on changes in behavior. In the present study, we examine how changes in phase-specific and generic health cognitions influence first, the adoption of exercise and later, the maintenance of the newly acquired behavior. Methods: Participants were middle-aged, Finnish adults participating in a lifestyle intervention to prevent Type 2 diabetes (N = 389). Health cognitions and exercise were measured at baseline (preintervention) and twice after a lifestyle intervention took place (at 3 months and after 1 year). Results: The results show that both phase-specific and generic health cognitions changed during the intervention. However, these changes were most pronounced within those participants who started at a lower level to begin with. Most important, evidence for the dynamic interplay between changes in health cognitions and health behavior was observed: Phase-specific health cognitions facilitated the adoption as well as the maintenance of behavior changes. Conclusions: Thus, health cognitions are amenable to change, and these changes are adaptive in terms of initiating and maintaining health behavior change. This has implications both for practical intervention research, as well as for theory development.