Nutritional disorders include a wide spectrum of conditions, including generalized under-nutrition, over-nutrition leading to obesity, the eating disorders and diseases where nutrition has a role in the aetiology. Both under-nutrition and obesity are important public health problems. The treatment of under-nutrition is often complicated by factors such as war famine and infectious diseases. Obesity remains difficult to treat once present, although the advances in the understanding of the physiology of feeding discussed in this chapter are leading to new pharmacological and surgical interventions. Nutritional interventions in the treatment of diseases may involve the use of therapeutic diets, the administration of dietary supplements or the provision of nutritional support, either enteral or parenteral. Nutritional support is best delivered by a multidisciplinary team. Further research is required to establish the efficacy of most dietary supplements.