Platelet aggregation is mediated by the platelet fibrinogen receptor, the alpha IIb beta 3 integrin (glycoprotein IIb-IIIa). This integrin has served as a prototype for studies probing structure-function relationships, cellular modulation of integrin function, and integrin-mediated signaling. Use of new model systems and molecular approaches have helped investigators to make rapid progress in our understanding of these areas, as reviewed here.