The immune system response to exercise is multifaceted, depending on the nature of exercise. The motivation behind recent research on the immune response to exercise comes from four basic directions: (i) keeping competitive athletes healthy, based on the perception that athletes are susceptible to infectious illness during intense training and competition; (ii) community interest in health promotion, based on the beneficial effect of exercise in lessening the risk of other lifestyle-associated diseases such as heart disease; (iii) use of exercise as adjunct therapy to improve patient functional capacity in certain diseases such as acquired immune deficiency syndrome (AIDS) and cancer; and (iv) work in the field of psychoneuroimmunology showing significant interaction between the neuroendocrine and immune systems, and the role of lifestyle factors in modulating immune function. Future work in exercise immunology will focus on topics such as: identifying the role of neuroendocrine factors in regulating the immune response to exercise; understanding the effects and appropriate type of moderate exercise in patients with diseases which involve the immune system; and possible long-term role of regular exercise in preventing diseases such as cancer, or the decline in immune function which accompanies the aging process.