Antiplatelet therapy with aspirin and a platelet P2Y12 receptor antagonist reduces thrombotic and ischemic events after percutaneous coronary intervention and acute coronary syndrome. The platelet inhibitory effect of the thienopyridine clopidogrel varies widely among individuals, and high on-treatment platelet reactivity has been associated with a substantial hazard for post-PCI cardiovascular events, including stent thrombosis. The clinical availability of ex vivo methods to measure the antiplatelet effect of P2Y12 antagonists raises the possibility that incorporating platelet function testing into clinical practice could facilitate a stratified and efficient approach to antiplatelet therapy following PCI, although data from definitive randomized trials supporting a routine approach are currently lacking.