This series of studies set out to investigate the effect of self-determination and the individual differences that are present in motivational orientation on exercise behaviour and the affective and motivational responses to exercise. Deci and Ryan's (1985a) self-determination theory (SDT) and its sub-theories, cognitive evaluation theory (CET) and causality orientations theory (COT) were used as the theoretical basis. The purpose of the research was to provide an indication of the exercise environment that will encourage the most positive responses and may promote the adoption and maintenance of regular exercise in individuals with different motivational orientations. The first study examined the effect of increased self-determination on the affective and motivational. responses to acute exercise. Results showed that increased self-determination made no difference to the affective response or to intrinsic motivation following exercise, although individuals chose to exercise at a higher intensity when given freedom of choice. Additional analyses showed that pre-exercise levels of affect influenced the response to exercise, as did individual differences with respect to motivational orientation. These individual differences were explained in terms of causality orientations and became the focus of the remainder of the thesis. Study two addressed the measurement of causality orientations specific to exercise. A measurement tool to assess causality orientations specific for exercise (the ECOS) was developed and was shown to be factorially valid and reliable and support was found for its concurrent validity. The third study was an intervention using the ECOS to investigate the interaction between causality orientations and the exercise environment on exercise behaviour. Psychological responses to regular exercise were measured at the situational and contextual level. Comparisons were made between individuals whose exercise environment was either supportive or not supportive of their predominant causality orientation and a control group. It was concluded that providing a matched exercise environment did not influence exercise behaviour. All individuals achieved and maintained the same levels of exercise. However, differences did emerge in psychological responses. Situationally, being autonomy oriented or in an autonomy supportive environment provided the most positive affective and motivational responses. Contextually, levels of autonomy, self-determined regulation and intrinsic motivation increased irrespective of causality orientation or exercise environment. Limitations of the research were discussed. Conclusions and future research based on an integration of the results of all three studies are presented with reference to SDT and COT along with the applied implications of the research with respect to exercise promotion.