This thesis examined the structure and nature of the relationships between physical activity and three components of mental well-being (physical, work-related, and global) in corporate and University employees. The first study tested a hierarchically based model which specified the interrelationships between exercise participation and indicators of the three components of well-being in a sample of corporate employees. Using Structural Equation Modelling (SEM), the results revealed a very good model fit. Exercise participation was directly related to indicators of physical well-being and enthusiasm at work, and indirectly with more global constructs of well-being (selfesteem, life satisfaction and job satisfaction). In order to examine the nature of these relationships further, Study 2 explored physical activity and well-being typologies using the same sample. A hierarchical cluster analysis revealed four distinct groups of employees, demonstrating the multi-dimensional nature of the relationship between physical activity and mental well-being. Using a qualitative approach, Study 3 sought to confirm the existence of the physical activity and mental well-being typologies, and explore potential psycho-social mechanisms linking physical activity participation with well-being. The results found general support for the existence of the profiles, and a range of possible psycho-social mechanisms, one of which was the regulation underlying the exercise behaviour. To explore the generalisability of the latter, Study 4 examined the role of exercise autonomy and its relationship with well-being using a large sample (N = 776) of University secretarial and administrative employees. The results provided support for the suggestion that the regulation underlying the exercise behaviour is important in understanding the relationship between physical activity and mental well-being. Finally, using the same sample, Study 5 examined well-being typologies of the physically inactive participants and performed a needs analysis for future exercise- and modular programmes in a new Centre for Sport, Exercise and Health in their workplace. This information may guide more cost-effective future interventions to promote activity and well-being in the workplace. The results of the present thesis are discussed in relation to implications for future practice and research directions.