Background: Exercise behaviour has largely been studied via reflective social cognitive approaches over the last thirty years. Emerging findings have shown habit to demonstrate predictive validity with physical activity. Habit represents an automatic behaviour that becomes developed from repeated stimulus-response bonds (cued and repetitive action) overtime. Despite the correlation with PA, the literature lacks research in understanding habit formation in new exercisers and experimental evidence of this construct. Hence, the purpose of this dissertation was to: i) understand the behavioural and psychological requirements of habit formation in new gym members, ii) investigate how regular gym members maintain their exercise habit, and iii) incorporate these findings to design a randomized-controlled trial (RCT) to test the effectiveness of an exercise habit building workshop in new gym members. In particular, the RCT sought to test if the habit group would develop greater exercise improvement over a control condition and another intervention group that employed a variety-based approach. Methods: Participants for all three studies were healthy adults (18-65) who were recruited from local gym and recreation centres in Victoria, BC. Studies I and III included only new gym members who were not meeting the Canadian Physical Activity guidelines upon recruitment while study II were a sample of gym members who have been exercising for at least one year. The first two studies were prospective, observational designs (twelve and six weeks respectively) while the third was a CONSORT based experimental study. Results: The first study found that exercising for at least four bouts per week for six weeks was the minimum requirement to establish an exercise habit. Trajectory change analysis revealed habit and intention to be parallel predictors of exercise in the trajectory analysis while consistency of practice revealed to be the best predictor. The second study highlighted the distinction between the preparatory and performance phases of exercise and further found intention and preparatory habit to be responsible for behaviour change across time. This study also found consistency to be the strongest predictor for habit formation. The intervention found the habit group to increase in exercise time compared to the control (p<.05, d=.40) and variety (p<.05, d=.36) groups. Mediation analysis found habit to partially mediate between group and behaviour. Contextual predictors revealed cues and consistency to mediate habit formation and group type. Conclusions: This dissertation provided significant novel contributions to the literature which included: i) calculating the behavioural and psychological requirements for establishing an exercise habit, ii) distinguishing two behavioural phases of exercise and iii) conducting the first exercise habit-based RCT. These findings demonstrate the effectiveness of the proposed habit-based worksheet which could be helpful for trainers and new gym members in facilitating an exercise habit. / Graduate / kaushal@uvic.ca