Studies on platelet membrane proteins during the past decade have defined a series of such components and their roles in key phenomena in hemostasis and thrombosis such as platelet adhesion and aggregation. Central among the membrane proteins is the glycoprotein group. At this time, several of these proteins have been characterized as to apparent molecular weights and subunit composition whereas sugar and amino acid composition and sequences await further studies for most of them. The absence of specific membrane glycoproteins in congenital bleeding diseases has helped to identify their functions as receptors for known ligands for binding reactions central to the hemostatic mechanism. Some of the membrane proteins may exist more or less loosely attached to each other on the surface of intact platelets, and this may be important for their functions.