Platelets localize, amplify, and sustain the coagulant response at an injury site and release procoagulant platelet-derived microparticles. Abnormalities in platelet size and function may be present in acute coronary syndromes and following percutaneous coronary intervention (PCI). Platelet functional assessment prior to PCI is a predictor for the subsequent occurrence of adverse clinical events. Trials of glycoprotein IIb-IIIa inhibitors as adjunctive pharmacotherapy for PCI show that the magnitude of platelet inhibition achieved by therapy correlates with the degree of clinical benefit observed. In general, optimal periprocedural outcomes require high levels of platelet inhibition. Combined administration of antiplatelet therapies may produce additive or synergistic inhibition of platelet-mediated thrombosis.