Platelets are key mediators in the pathophysiology of atherothrombotic disease. Inappropriate platelet activation can lead to thrombosis and ischemic events. Platelet function testing has been used to monitor patient response to antiplatelet therapy. Variability in response to antiplatelet therapy may be due in part to incomplete inhibition of targeted pathways (thromboxane A(2) and adenosine diphosphate [ADP]) by currently available agents (aspirin and P2Y(12) ADP receptor antagonists). Low responsiveness to antiplatelet therapy (as measured in various platelet function assays) correlates with a high rate of ischemic events. However, tailoring treatment based on platelet response remains to be definitively proven in clinical trials, correlations between assays are modest, and concordance in defining suboptimal response is poor. Additional studies are needed to determine whether changes in therapy based on results of platelet function testing improve clinical outcomes, and thus will determine whether broader use of platelet function testing in clinical practice is warranted.