Aspirin is an effective antiplatelet drug for preventing thrombo-embolic vascular events. However, clinical and laboratory evidence demonstrates diminished or no response to aspirin in some patients that is called aspirin resistance. This situation has been reported to be independently associated with an increased risk of adverse cardiovascular events. The exact mechanism of aspirin resistance has not been established yet. The clinical, pharmacological and genetic factors may be associated with aspirin resistance. However, there is not currently standardized test to the diagnosis and no proven effective treatment of aspirin resistance. This article summarizes aspirin resistance, discussing its definition, clinical outcomes, laboratory tests, possible causes and therapeutic approaches.