Platelets play a central role in the pathogenesis of atherothrombosis. Dual antiplatelet therapy with clopidogrel plusaspirin has been shown to reduce ischemic events in patients undergoing percutaneous coronary intervention(PCI) and stenting. Although dual antiplatelet therapy reduces the risk of cardiovascular episodes after PCIs, a substantialnumber of incidents continue to occur. Many cardiologists have focused their attention to the relationshipsbetween the interindividual variability of platelet inhibition after aspirin or clopidogrel administration and majorcardiac adverse events such as stent thrombosis. Recent evidence has suggested that “aspirin or clopidogrel resistance”is associated with poor health outcomes (recurrent atherothrombotic events and stent thrombosis) after drugeluting stent (DES) implantation. However, the current clinical guidelines do not support routine screenings for antiplateletresistance because standardized objective screening has not yet been established. Thus, this review describesthe antiplatelet therapy used in PCI and it outlines the mechanism, laboratory tests, clinical impact and treatmentoptions for aspirin and clopidogrel resistance in the DES era. (Korean Circulation J 2007;37:135-147)