As more is understood about the physiology of exercise, both in normal and in diabetic subjects, its role in the treatment of diabetes is becoming better defined. Although persons with diabetes may derive many benefits from regular physical exercise, there also are a number of hazards that make exercise difficult to manage. In insulin-treated diabetics, there are risks of hypoglycemia during or after exercise or of worsening metabolic control if insulin deficiency is present. Type II diabetics being treated with sulfonylureas also are at some increased risk of developing hypoglycemia during or following exercise, although this is less of a problem than occurs with insulin treatment. In individuals treated by diet alone, regulation of blood glucose during exercise usually results in a decrease in glucose concentration toward normal but not to hypoglycemic levels and exercise can be used safely as an adjunct to diet to achieve weight loss and improved insulin sensitivity. When obese patients with type II diabetes are treated with very low calorie diets, adequate amounts of carbohydrate must be provided to ensure maintenance of normal muscle glycogen content, particularly if individuals wish to participate in high intensity exercise that places a heavy workload on specific muscle groups. On the other hand, moderate intensity exercise such as vigorous walking can be tolerated by individuals on very low calorie, carbohydrate-restricted diets after an appropriate period of adaptation. A number of strategies can be employed to avoid hypoglycemia in patients with insulin-treated diabetes and both type I and type II diabetic subjects should be examined carefully for long term complications of their disease, which may be worsened by exercise. These considerations have led many diabetologists to consider exercise to be beneficial in the management of diabetes for some individuals but not to be recommended for everyone as a "necessary" part of diabetic treatment as was thought in the past. Instead, the goals should be to teach patients to incorporate exercise into their daily lives if they wish and to develop strategies to avoid the complications of exercise. The rationale for the use of exercise as part of the treatment program in type II diabetes is much clearer and regular exercise may be prescribed as an adjunct to caloric restriction for weight reduction and as a means of improving insulin sensitivity in the obese, insulin-resistant individual.