Chronic diseases such as type 2 diabetes are increasing worldwide. Many people suffer from complications that lower their quality of life and life expectancy. Healthy eating and sufficient physical activity are the best and the most costeffective means of preventing and delaying such problems. However, adopting and maintaining a healthful diet and regular exercise is not an easy task even for well-motivated individuals, as it requires many psychosocial changes during the lifestyle change process.

This dissertation focuses on psychosocial factors related to successful behaviour change in the context of a lifestyle intervention to prevent type 2 diabetes and examines whether these processes differ between genders and socioeconomic groups. In addition to the key modifiable psychosocial predictors - self-efficacy, action and coping planning, and social support - the influence of personality is also investigated. The dissertation offers insights into developing and planning evidence-based behaviour change interventions in practice, and implications for the science of behaviour change applied in health promotion as well as for personality and social psychology.