Adhesion molecules expressed on leukocytes and the vascular endothelial lining include the selectins, integrins, members of the immunoglobulin superfamily, and mucins. The changes in their expression that develop with acute and chronic exercise are briefly reviewed. Adhesion molecules are thought to modulate leukocyte trafficking, accounting for changes in the counts and possibly also the functional activity of various leukocyte subsets during and following an acute bout of physical activity. Some of the changes in the surface density of adhesion molecules can be explained through the action of epinephrine and other humoral factors on their expression, but an influence of sympathetic nerve terminals on cells sequestered in the spleen and liver, and an influx into the general circulation of leukocytes of differing phenotype also appear to be involved.