It is difficult to define what is meant by the term obesity in elderly persons (EPs), as aging is associated with increased body fat and reduced lean body mass. The body mass index (BMI) is thus difficult to interpret, particularly as height also decreases with age. Thus, the percentage of body fat in an EP is around the same level as the threshold at which a young adult would be considered obese (25% for men and 35% for women in the over 60s). Moreover, it would seem important to consider body fat distribution and fat-free mass are situated, rather than just the BMI; in this way, three clinical forms of obesity can be described: abdominal obesity (or central obesity), age-related lipodystrophy and sarcopenia, which all need careful assessment in terms of prognosis. The various complications of obesity are presented, as obesity can cause numerous functional disorders and is a risk factor for metabolic and cardiovascular disease as well as for cancer and cognitive decline. Nevertheless, excess adipose tissue can protect against osteoporosis and wasting disease.