In transfusion medicine, platelets cannot be replaced by blood substitutes. Circulating platelets must respond quickly to changes in normal blood flow and blood-vessel injury to promote normal hemostasis. Adhesion of platelets at the site of vessel endothelial rupture is mediated through platelet membrane glycoprotein receptors. The integrity of these surface adhesion receptors and the signal-transduction pathways of activation will determine, in large part, how well a platelet functions in hemostasis. The deterioration of these systems during storage leads to a compromise of function known as the 'platelet-storage lesion'.