Pysical exercise is essential for maintaining normal function of skeletal muscle. Muscle tissue also has a remarkable capacity for adaptation to changes in physical demand. In fact, without stimulation from physical activity, muscle tissue will atrophy. The mechanisms responsible for increases or decreases in muscle function are to a large extent not known. According to current opinions, one consequence of physical exercise can be muscle cell damage and inflammation. The inflammatory process is suggested to be one mechanism for muscle adaptation to exercise. Direct evidences for exercise-induced muscle inflammation in humans are weak. Nevertheless, the immune system seems to be of great importance for muscle adaptation. The capacity of the muscle tissue to adapt is largely due to the existence of satellite cells and local growth factors, but the exact molecular mechanism has not been discovered. An interaction between known and yet undiscovered factors are most likely involved m the adaptation process. Physical exercise will also change the number, activation and function of circulating leukocytes. Some of these changes are mediated via adhesion molecules, cytokines, growth factors and hormones.