Combined inhibition of platelet aggregation is essential to prevent recurrent ischemic episodes in patients with acute coronary syndromes and after percutaneous coronary interventions (PCIs). In combination with aspirin, the ADP receptor antagonist clopidogrel is used widespread for this purpose; however, platelet reactivity after clopidogrel differs substantially between patients and high on-treatment platelet reactivity (HTPR) persists in a substantial proportion of cases. Since more than 20 prior observational studies linked HTPR to higher risk of ischemic events, including cardiovascular death, myocardial infarction, and stent thrombosis, monitoring post-clopidogrel platelet reactivity after PCI might be beneficial for risk assessment and to tailor the antiplatelet therapy to the patients' needs. However, there is no consensus on the role of routine platelet function monitoring in clinical guidelines. This article aims to review the available evidence regarding the clinical relevance of platelet function monitoring, highlighting possible reasons for the controversy between guidelines and observational studies.