The transtheoretical model has been used to understand the stages individuals progress through, and the cognitive and behavioral processes they use while changing health behaviors. The model postulates that individuals engaging in a new behavior move through the stages of Precontemplation, Contemplation, Preparation, Action, and Maintenance. Movement through these stages does not always occur in a linear manner, but may also be cyclical as many individuals must make several attempts at behavior change before their goals are realized. The amount of progress people make as a result of intervention tends to be a function of the stage they are in at the start of treatment. Instruments have been developed to measure the stages and processes of exercise adoption and maintenance and the related constructs of exercise specific self-efficacy and decision making. Psychometric data on these instruments are reviewed. Additionally, data collected on these measures from worksites in the U.S. and Australia are presented along with data from interventions aimed at increasing the adoption of physical activity among community volunteers and worksite employees. Applications of the transtheoretical model for the initiation, adoption, and maintenance of exercise behavior from clinical, community, and public health perspectives are discussed.