Most lifestyle interventions fail at instilling behavioral maintenance. This is likely a result of interventions being based on suboptimal theories, the majority of which focus exclusively on determinants of behavioral intentions. These theories and interventions based on them are limited in their ability to explain and promote, respectively, behavioral initiation and maintenance. First, fewer than 50% of people translate their intentions into behavior; second, only mechanisms of initiation are specified while mechanisms of maintenance are neglected. Thus, more basic, theoretical work regarding mechanisms of behavioral initiation (i.e. moderators of the intention-behavior relationship) and maintenance is required to advance health practices. The purpose of the present line of research was to (1) evaluate a contemporary theoretical framework—the Integrated Behavior Change Model (IBCM)—that claims to better account for behavioral initiation, which requires more research evidence; and (2) to propose and test the utility of an extension to the IBCM to more fully account for mechanisms of behavioral maintenance, within the context of exercise. A secondary purpose of the present study was to test whether mechanisms of behavioral engagement vary for people who are just starting to exercise (i.e., initiators) versus those who have been exercising from some time (i.e., maintainers). Results supported hypotheses that the original IBCM would be more relevant for initiators versus maintainers, but counter to hypotheses, the ICBM did not do a better job at predicting behavior than its theoretical predecessor - the Theory of Planned Behavior. Assessment of the extension of the IBCM with maintainers revealed that identity was a stronger predictor of behavior than habit and that identity was particularly important when people’s routines varied from day-to-day, as expected. These results highlight the continued need for theoretical refinement and development in the field of Health Psychology applied to behavior change interventions.