Muscle atrophy is defined as a decrease in muscle mass, cross-sectional area, and myofibrillar protein content. Causes inducing muscle atrophy may be inactivity, denervation, undernutrition and steroid. Inactivity may decrease protein synthesis and increase protein breakdown of skeletal muscle. The muscle atrophy due to inactivity was induced by bed rest, hindlimb suspension, cast, total hip replacement arthroplasty, anterior cruciate ligament reconstruction. Denervated atrophy may be induced by the loss of innervation from lower motor neuron. The atrophy was apparent in the lower limb of hemiplegic patients following ischemic stroke and in the hindlimb of ischemic stroke rats. Protein breakdown of skeletal muscle in the undernourished state results in muscle atrophy. The atrophy due to undernutrition was evident in cancer and leukemia patients and in the undernourished rats. Steroids have been used to treat allergies, inflammatory diseases, autoimmune diseases and to inhibit immune function following transplantation. Steroids may induce muscle atrophy by protein breakdown of skeletal muscle. Muscle Physiology Laboratoryat College of Nursing, Seoul National University proved that dexamethasone may induce hindlimb muscle atrophy in rats and exercise and DHEA may attenuate hindlimb muscle atrophy induced by the steroid in rats. Nurses working with patients undergoing steroid treatment need to be cognizant of steroid induced muscle atrophy. They need to assess whether muscle atrophy is being occurred during and after the steroid treatment. Moreover, they need to apply exercise and DHEA to the patients undergoing steroid treatment in order to attenuate the steroid induced muscle atrophy.