PECAM-1/CD31 represents a new addition to the cell adhesion molecules of the Ig superfamily. Recent work has revealed that it is capable of complex ligand interactions, although the specific ligands involved are still unknown. The wide distribution of PECAM-1 among vascular associated cells suggests that it may have number of important physiological functions. The ability of anti-PECAM-1 antibodies to block normal endothelial cell-cell contacts and influence cell migration point to a role in angiogenesis and wound healing. PECAM-1 may also contribute to early cardiovascular development. Augmentation of integrin-mediated white blood cell adhesion by engagement of PECAM-1 suggests that it may be involved in the adhesion of leukocytes to the endothelium and thus participate in the inflammatory response. Its function on platelets, however, still remains to be determined. Activational events are probably required in vivo for the molecule to function, given the high levels of constitutive expression. Phosphorylation of the cytoplasmic domain may be one of these events. It is anticipated that, as our understanding of the molecular and functional properties of PECAM-1 grows, we will gain new insights into the processes of inflammation, wound healing and angiogenesis.