Eating disorders describe a range of conditions that are defined by abnormal eating habits that involve either inadequate or excessive nutrition to the detriment of an individual's mental and physical health. This chapter focuses on the pathology of body weight, immunity and gut function in eating disorders, exploring the possible role of neuropeptides and discussing the potential role for probiotics in moderating these systems within eating disorders. The effects of Bulimia Nervosa (BN) and Binge eating disorder on immunity are less well studied. Patients with BN do not become as emaciated as patients with AN, but their weight often fluctuates >5 kg, due to the alternating binge eating and fasting or purging behaviour. Functional gastrointestinal disorders are common in eating disorder patients, and in the majority of patients these persist even after the recovery from eating disorders, particularly in psychologically distressed patients. Neuropeptides may well have a significant part to play in the pathogenesis of eating disorders.