We applied the model of the stages and processes of change to exercise adoption and maintenance. This model has shown promise in advancing theory and treatment of the addictions and other negative health behaviors, but there have been few systematic attempts to apply the model to positive health behaviors, such as exercise adoption. Questionnaires dealing with the stages and processes of exercise change were developed and administered to a sample of 1,172 participants in a worksite health promotion project. The sample was split randomly into halves for (a) initial model development and testing and (b) confirmatory measurement model testing. Additional model confirmation was obtained by examining the hierarchical structure of the processes of change and by conducting Stage x Process analyses. Results suggest that the underlying constructs derived from smoking cessation and other addictive behaviors can be generalized to exercise behavior. Understanding the stages and processes of exercise behavior change may yield important information for enhancing exercise adoption, adherence, and relapse prevention at both individual and public health levels.