Despite the application of new antiplatelet drugs (prasugrel and ticagrelor), dual antiplatelet therapy with clopidogrel and aspirin remains the standard for patients with acute coronary syndrome undergoing percutaneous coronary intervention, especially in countries of low socioeconomic status. Regardless of the proven benefits, numerous studies have shown that certain groups of patients who receive standard doses of clopidogrel and aspirin do not respond adequately, and many of them also exhibit adverse cardiovascular events. Studies have shown that the risk of stent thrombosis and ischemic complications is higher in patients with: acute coronary syndrome, diabetes mellitus, thrombocytosis, reduced systolic function of the left ventricle with ejection fraction less than 30%, presence of multiple stents, longer and thinner stents, and renal failure. In these patients it is particularly important to assess the response to clopidogrel and selecting adequate antiplatelet therapy; this provides an impetus for platelet function tests. The second especially significant group to target for laboratory evaluation includes patients with increased risk of bleeding, such as elderly patients, patients with low body weight, anemia, thrombocytopenia, renal failure, past or current ventricular or duodenal ulcer, coagulopathy, or liver disease. The third important application of platelet function tests entails the preparation and evaluation of the time for surgical interventions or invasive diagnostic procedures in patients on antiplatelet therapy. These tests can also be helpful for monitoring the effects of therapy of bleeding due to platelet dysfunction. For high-risk patients the careful selection of optimal antiplatelet drug(s) on the basis of estimated individual risk of thrombosis and bleeding, pharmacodynamic characteristics of each drug, and patien's comorbidity remains essential.