Undernutrition is common in older people and has serious adverse effects. Weight loss and low body weight are key markers. Correctable causes, such as depression, are common and should be sought. Structured efforts to encourage food intake, together with nutritional supplements, often are of benefit. It is hoped that a better understanding of the underlying mechanisms will lead to targeted treatments. Overweight and obesity also are common in older people, and are associated with morbidity and impaired function. It is probably appropriate to recommend weight loss to obese older people who have associated comorbidities, particularly reduced mobility, but seldom, if ever, for increased weight alone.