Patients with acute coronary syndromes and patients who undergo coronary stent implantation frequently receive dual antiplatelet therapy with aspirin and a thienopyridine. A wide variability exists in the inhibitory response to the thienopyridine clopidogrel among individuals, but this variability can be detected by a variety of platelet function assays. Several studies have demonstrated an association between high platelet reactivity post clopidogrel and subsequent cardiac events. To be adopted into routine clinical practice, platelet function assays must not only be able to identify at-risk patients, but they must also determine the appropriate thresholds for nonresponsiveness that provide appropriate sensitivity and specificity for subsequent adverse events. Prospective, randomized trials should determine whether therapeutic interventions guided by platelet function tests can safely and effectively reduce thrombotic events in at-risk patients. The use of platelet function monitoring to guide the timing of surgery after the discontinuation of antiplatelet therapy also merits further study.