Aspirin (acetylsalicylic acid) is a powerful antiplatelet agent used in prevention of atherothrombotic vascular events. However, antiplatelet effect of aspirin is not uniform and some patients could not benefit from aspirin. These patients are clinically called as aspirin resistant or aspirin non-responders. Aspirin resistance could be determined by: bleeding time, optical aggregometry, PFA-100 (Platelet Function Analyzer), Ultegra-RPFA (Rapid Platelet Function Assay), activated aggregation time, whole blood aggregometry, platelet aggregate ratio, flow cytometry, measurements of platelet surface proteins and blood or urine thromboxane 132 levels. Mechanisms of aspirin resistance have not been elucidated yet. There is evidence that aspirin resistance increases clinical cardiovascular events. Adequate additional therapies may reduce atherothrombotic risks and major cardiovascular events rate in aspirin resistant subjects. However, we need further studies to decrease major cardiovascular events risk in aspirin resistant subjects and to optimize antiplatelet therapy.