Lymphocytes recirculate extensively throughout the body and then localize in tissues and lymphoid organs. This is accomplished by an array of adhesion molecules on lymphocytes and counter-receptors on the vascular endothelium, extracellular matrix and the epithelium. Recent studies have identified several of the specific molecular interactions that mediate lymphocyte trafficking. Lymphocytes expressing alpha 4 beta 7 home to the intestine through recognition of the mucosal addressin cell adhesion molecule-1 (MAdCAM-1) expressed on Peyer's patch high endothelial venules. T cells in nearly all mucosal epithelial sites express the alpha E beta 7 integrin which has been shown to mediate lymphocyte binding to epithelial cell E-cadherin. Thus, members of the B7 subfamily of integrins, alpha E beta 7 and alpha 4 beta 7, and their counter-receptors, are believed to play key roles in directing lymphocyte traffic to and retention in mucosal organs.