One hundred twenty-seven years after Professor Giulio Bizzozero described the blood particle that has come to be known as the platelet, antiplatelet therapy has revolutionized the treatment of cardiovascular disease. Platelet function testing, introduced in 1962 with the advent of Born's aggregometer, heralded a renaissance in platelet research and provided a platelet function test to study platelet reactivity in vitro to help the diagnosis of bleeding disorders. More devices to test platelet function have emerged since, and these are now being applied mainly to assess antiplatelet drug efficacy in thrombotic disorders. Although this may be a logical use for platelet function tests, the data are replete with contradictions, and there is a lack of both consensus and standardization of the methodology. As a result, the clinical validity of platelet function results to monitor response to antiplatelet drugs has yet to be established.