Change in the proportion and functional activity of immune cells with exercise and training is reviewed in this article. A schema is proposed to categorize the initial event responsible for the exercise-induced mobilization and activation of leukocytes as either mechanical or metabolic. Irrespective of the initiating event, the phenomenon of a biphasic leukocytosis after exercise, and its hormonal regulation, has been well characterized. Despite some conflicting evidence, it appears that moderate exercise may cause a modest and transient enhancement of immunity, whereas intensive exercise may suppress it. Whether these changes are of biologic significance is yet to be determined. Interpretation of these studies is complicated by large variability in experimental design and treatment. With current knowledge, the physician and coach can implement several strategies for the prevention and management of illness in the elite athlete.