The study will examine the mediating effect of theoretical constructs based on the Transtheoretical Model (TTM) and Social Cognitive Theory (SCT). The study will address the methodological, measurement, and statistical limitations of previous studies. The investigators will randomly assign 420 healthy sedentary individuals to either a theory-based physical activity intervention or a contact control group. Physical activity behavior and the potential mediators will be assessed monthly over 12 months. They hypothesize that constructs based on the TTM and SCT (i.e., behavioral processes, cognitive processes, self-efficacy, and decisional balance) will mediate the relationship between group assignment (intervention vs. contact control) and physical activity behavior, such that the effect of the intervention on physical activity behavior relative to the contact control group will be attenuated when controlling for the mediators. They will also examine the importance of particular mediators for different population groups (e.g., age, gender), which will help to create more effective interventions for these groups. Finally, they will conduct a cost analysis to not only inform policy makers of the cost of delivering the intervention within our study but also to estimate the cost of delivering each component of the intervention (i.e., mediator). The analysis will estimate the cost of future refined interventions that incorporate those active components that affect the mediators and physical activity in this study. In summary, the study will identify which mediators (i.e., theoretical constructs) are most effective for increasing physical activity, which will have an important public health impact given that this will likely lead to more efficacious and less costly interventions.