This research examined the processes underpinning changes in psychological well-being and behavioural regulation in Cardiac Rehabilitation (CR) patients using Self-Determination Theory (SDT; Deci & Ryan, 1985). A repeated measures design was used to identify the longitudinal relationships between SDT variables, psychological well-being and exercise behaviour during and following a structured CR programme. Participants were 389 cardiac patients (aged 36-84 years; M age = 64 ± 9 years; 34.3% female) referred to a 12 week supervised CR programme. Psychological need satisfaction, behavioural regulation, healthrelated quality of life, physical self-worth, anxiety and depression were measured at programme entry, exit and 6 month post-programme. During the programme, increases in autonomy satisfaction predicted positive changes in behavioural regulation, and improvements in competence and relatedness satisfaction predicted improvements in behavioural regulation and well-being. Competence satisfaction also positively predicted habitual physical activity. Decreases in external regulation and, increases in intrinsic motivation, predicted improvements in physical self-worth and physical well-being respectively. Significant longitudinal relationships were identified whereby changes during the programme predicted changes in habitual physical activity and the mental quality of life from exit to 6 month follow-up. Findings provide insight into the factors explaining psychological changes seen during CR. They highlight the importance of increasing patients' perceptions of psychological need satisfaction and self-determined motivation to improve well-being during the structured component of a CR programme and longer-term physical activity.3