P-selectin and PSGL-1 are cell adhesion molecules, regulating the initial interactions between leukocytes and the blood vessel wall as well as between activated platelets and leukocytes. P-selectin is expressed on activated endothelial cells and platelets, while its major ligand PSGL-1 is expressed on leukocytes. Multiple studies have shown that these adhesion molecules are required for the normal recruitment of leukocytes during an inflammatory reaction. More recently, these adhesion molecules have been implicated in recruitment of leukocytes and leukocyte microparticles to thrombi. The P-selectin- and PSGL-1-dependent delivery of circulating microparticles to thrombi appears to be important for normal tissue factor accumulation and fibrin generation in thrombi.