If type 1 diabetic patients take too little insulin, exercise accelerates the rate at which hyperglycemia and ketosis develop. On the other hand, if the patients take too much insulin, hypoglycemia develops more rapidly, if they also exercise. However, education and home monitoring of blood glucose concentrations allow insulin‐treated diabetic patients to change amount and timing of insulin and food intake to cope with any exercise challenge. In type 2 diabetic patients the metabolic risks with exercise are less. In the postprandial state exercise always reduces glucose levels, and the decrease seems determined primarily by overall energy expenditure. In response to regularly repeated exercise, i.e. training, type 2 diabetic patients show an increase in insulin sensitivity in muscle and a reduction in HbA
1c
levels. In contrast, type 1 diabetic patients cannot be recommended training as a means to improve glycemic control. However, in addition to mental and social benefits both groups may achieve improvements in aerobic exercise capacity, muscle strength, arterial blood pressure, and endothelium‐dependent vasodilation by training. Furthermore, cardiovascular morbidity and mortality are lower and life expectancy increased in physically active compared with sedentary diabetic patients. The risk of getting type 2 diabetes can also be reduced by an active life style. Before increasing the level of physical activity all diabetic patients should undergo a thorough medical evaluation. In case of overt or suspected clinically silent long‐term complications the examination should include a graded exercise test with ECG and blood pressure measurement. Based on the examination an appropriate program, which accounts for individual limitations and preferences and includes, for instance, both aerobic and resistance training, can be worked out for all patients.