Muscle strength decreases with increasing age. The main causes for this are intrinsic age-dependent mechanisms and hypokinesia. Immobilization and bed rest accelerate the loss of muscle strength. A number of studies show that even very old people can increase their strength, sometimes to a surprising degree. Thus, there is considerable potential for strength training in the rehabilitation of the elderly. Adaptation to strength training, the methods for improving strength and the importance of health problems related to strength training are discussed. If carefully planned and supervised, strength training is feasible even in people with chronic diseases like mild hypertension, coronary heart disease and osteoarthritis; actually, strength training may even be beneficial. Also, strength training becomes increasingly important in rehabilitation after orthopedic surgery.