Background The thesis consists of a series of studies relating to physical activity in adults resident in Wirral, North West England. The first study is a survey of the health and lifestyles of the general adult population, which sets the context for the other studies. The subsequent studies focus on a specific section of this population. These were adults who were at risk of coronary heart disease who had been referred to a primary care exercise referral scheme (called the Exercise and Lifestyle Centres or ELC). The research adopted health locus of control (HLC) and Wallston's modified social learning theory (MSL T) as a theoretical perspective in order to explore physical activity behaviour (Wallston, 1992). A self-regulatory model of health behaviour was also found to be a useful framework for explaining concepts that emerged from the studies (Leventhal, Diefenbach & Leventhal, 1992). Aims The aim of the thesis was to investigate factors that are associated with, and have an impact on physical activity behaviour. In particular, it sought to explore and understand factors that influence the initiation and maintenance of activity in individuals referred into the ELC scheme. It also set out to investigate the usefulness ofHLC and Wallston's MSLT in helping to understand physical activity in this population group. Methodology Quantitative methods included a large scale survey of a sample of the Wirral population, an evaluation of the ELC scheme and a study of Walls ton's MSLT. Findings were analysed using chi square analysis, logistic regression and structural equation modelling. Qualitative methods included semi-structured interviews with thirty two individualswho were referred to the ELC. An adaptation of grounded theory methodology was used as advocated by Smith (2001), Charmaz (2001) and / described by Bennett and Vidal-Hall (2000). Findings Factors found to influence physical activity behaviour included age; gender; perceptions of health status; illness representations; beliefs about capability to participate in physical activity; beliefs about the pros and cons of engaging in activity (such as health benefits, enjoyment and social interaction versus risk of injury, lack of time and competing demands of work and family); beliefs about whether own actions would have an influence on valued outcomes; receptivity to the health message from General Practitioners (GPs); time perspective with respect to illness representations and the value placed on the longer-term benefits of physical activity. Discussion There was a complex relationship between the factors that were found to influence the initiation and maintenance of activity. Age was particularly influential and appeared to have a mediating influence on health status, illness representations, selfefficacy, health value, HLC beliefs and barriers towards activity (such as fear of injury and perceptions of enjoyment). Gender was also a key influence on the initiation of activity ~ Having a combination of internal HLC and powerful other HLC beliefs was particularly important in initiating activity on the ELC. Other factors that contributed to adherence to the ELC included enjoyment, obtaining valued outcomes and positive perceptions of self-efficacy. However, maintenance of activity beyond the ELC was positively influenced by a time perspective that viewed health as a long-term valued outcome, that could be achieved via continued physical activity behaviour, which was found to be within capabilities to carry out and enjoyable. Reeemmendatiens Adopting a one sizefits all strategy will not be very effective in increasing levels of physical activity. Support needs to be focused towards the age and gender profile of participants, their health conditions, particular barriers and concerns, as well as their psychological preferences and perspectives (e.g. their HLC beliefs, perceptions of capability and readiness to change). . Conclusion Physical activity behaviour is both complex and multi-determined. Constructs from different models need to be integrated in order to understand physical activity. Qualitative methods are particularly helpful in understanding the differences between those who participate in a primary care exercise referral scheme and those who do not.