The aim of the thesis was first of all to examine the relationships between the constructs of self-determination theory and motivational processes and outcomes within the GP exercise referral population; and secondly, to determine whether the practice of motivational interviewing can be understood within the unifying theoretical framework of Deci and Ryan' s (1985) self-determination theory. Four studies were conducted in order to obtain these global aims. The first study involved the adaptation and development of an instrument to measure perceptions of environmental support and psychological need satisfaction. This study produced a factorially valid measurement tool, and structural equation modelling analyses supported the hypothesised model with environmental supports having a moderately strong effect upon need satisfaction, which in tum had a strong effect upon relative autonomous regulation. The second study examined the relationships between perceived environmental support, perceived psychological need satisfaction, and exercise adherence longitudinally. Results suggested that the degree of self-determination experienced by the participants mid-way through the exercise referral scheme was an important predictor of exercise maintenance three-months after the scheme had finished. The study also showed how autonomy and relatedness play an important role in the internalisation of exercise behaviour; specifically, when individuals experience feelings of autonomy, competence, and relatedness in the exercise environment can lead to higher levels of intrinsic and identified regulation and lower levels of amotivation, introjected, and external regulations are observed three-months post-scheme. The third study was a cross-sectional design, and assessed the individual's 'situational' experiences of environmental support (autonomy support, structure, and involvement) and the corresponding perceptions of autonomy, competence and relatedness within an exercise class. Results revealed that even in a situational context, the conceptual model developed in the first study (i.e. the contextual study) was upheld. The final study compared a motivational interviewing-based intervention with an attention control group (educational information sessions) and a control group in order to tease apart the effect of motivational interviewing upon the dependent variables, and the effect of attention on the dependent variables. Results revealed that the individuals receiving the motivational interviewing based intervention possessed significantly greater levels of autonomy support than the control group, and lower levels of post-scheme amotivation.