Physical activity appears to be one of the behaviours of patients with anorexia nervosa that may worsen over time and contributes to maintaining the dieting disorder. Seasonal variation in physical activity has been demonstrated. The current thesis therefore addresses three main questions: a) do Australian and German female students perceive seasonal variation in physical activity in a similar manner b) does physical activity vary across the seasons in female patients with anorexia nervosa compared to healthy female controls and c) what assessment and treatment strategies are implemented by clinical specialists? In summary, seasonal variation in physical activity was reported by Australian and German students. Further, it was demonstrated that time spent in some components of physical activity (moderate intensity activities and exercise) varied across the seasons in the patient-group. Comprehensive assessment tools and published exercise programmes were rarely used by the clinical specialists, indicating the need to extend treatment guidelines to include physical activity and its seasonal variation.