The understanding of the structure and function of platelet membrane glycoproteins has been facilitated by studies showing that they belong to larger gene families of cell surface receptors involved in cellular interactions. In some instances (e.g. GP IIb-IIIa and GP Ib-IX) the study of the platelet proteins has served as a prototype for relatively newly described gene families (e.g. integrins and LRG proteins, respectively). In other instances, e.g. PECAM-1, the background of information on immunoglobulin domain-containing proteins has served to indicate functions. Receptor-ligand interactions have been characterized at the molecular level, and studies of genetic defects affecting platelet receptors have contributed significantly to understanding structure-function relationships. Gene transfection studies provide encouraging results that might lead to gene therapy. The knowledge about platelet ligand-receptor processes contributes not only to our understanding of normal platelet function, but also to a more generalized understanding of adhesive mechanisms used by many cells to interact with their environment.