Physical inactivity is widely recognised as a major threat to the health and well-being of populations in developed countries. Currently the number of physically inactive persons, regardless of age and gender, appears to increase. Therefore, the development of promotional strategies to en-hance physical activity levels of populations is well founded. The present work is a comprehensive analysis of conceptual, physiological and behavioural data concerning health-enhancing physical activity. Behavioural change related to physical activity is also introduced. The emphasis is on ecological models of health behaviour - an approach which takes individual, social, and physical environmental factors into account.The purpose of the empirical part of the work is to examine the role of factors highlighted by the ecological models on physical activity behaviour, using running as the behavioural model. By focusing on one specific type of activity, it is possible to analyse the significance of different factors on physical activity in a well controlled setting.539 middle-aged participants of a women's fun run responded to two consecutive questionnaires, administered two years apart. Logistic regression analysis was used to analyse the prediction of leisure-time running behaviour during the follow-up. Controlled for age, education, living situation and place of residence, the analysis showed that the frequent use of behavioural techniques and a high enjoyment of running, in cases of low family support, predicted the adoption of regular running. There was an interaction between the enjoyment of running and family support: those who rated enjoyment of running high and reported high family support were less likely to adopt running. With regard to regression from running, logistic regression analysis indicated that the infrequent use of motives and poor perceived health, in cases of an attractive neighbourhood, predicted regression from regular running. There was an interaction between poorly perceived health and an unattractive neighbourhood: those who perceived themselves to be in poor health and had an unattractive neighbourhood were more likely to regress from regular running.This book provides a multi-disciplinary knowledge base for planning, developing and evaluating theory-based physical activity programmes in the public health setting.