Physical activity (PA) is a modifiable behaviour that carries implications for the mental health of the UK. Theory-driven research highlights that the reasons why we participate in PA, not just the act of participation, carry important consequences for human psychological growth, optimal functioning and well-being. Implementing cross-sectional, longitudinal, and experimental designs, this thesis, tests the application and predictive utility of Selfdetermination theory in the health domain of PA behaviour change. Results highlight that the social-environment surrounding individuals before, during and after an exercise referral programme, along with one’s motivational regulations, have important consequences for intentions to be physically active and mental well-being. A SDT-based observational instrument operationalised the environment afforded by exercise professionals during one-to-one PA interactions. This more objective measure may provide further insight into the psychological processes responsible for behavioural and psychological outcomes. Finally, this thesis commences investigations into the moderating role of motivation on the limited self-control resource and the ability to employ counteractive control strategies that help one to achieve important health goals. Knowledge of the processes responsible for psychological health and behavioural intentions before, during and after PA interventions along with the ability to employ self-control, may carry important practical implications for future PA interventions.