The reactivity of platelets to agonists plays a central role in the genesis of thrombosis following percutaneous coronary intervention (PCI) and spontaneous plaque rupture. Antiplatelet therapy has reduced the occurrence of thrombotic events following PCI, including myocardial infarction and stent thrombosis. Because the platelet is a fundamental component in the generation of an arterial thrombus and the stimulus for thrombosis is marked in PCI, it is logical to predict that patients with superior platelet inhibition would have the best outcomes with respect to ischemic events post PCI. However, until recently there was little information linking measurements of high ex vivo platelet reactivity to the occurrence of ischemic events. An emerging body of data, mostly from small studies, supports the pivotal link between periprocedural platelet physiology and increased risk of adverse thrombotic events. This review explores the available information linking platelet reactivity during and after PCI to the occurrence of adverse events, including myocardial infarction, recurrent ischemia within 6 months, and stent thrombosis.