Aspirin is a non-steroidal anti-inflammatory drug that has potent antiplatelet actions. Aspirin was initially used as an analgesic and antipyretic drug before its anti-inflammatory properties were discovered. Aspirin also has antithrombotic effects due to the inhibition of cyclo-oxygenase activity in platelets, which reduces the extent of thromboxane A2 formation and consequently the aggregability of platelets. Prophylactic low-dose aspirin therapy reduces the risk of future cardiovascular events in a variety of clinical settings. The maximum effect of aspirin in reducing risks of myocardial infarction is achieved soon after the initiation of therapy. The selective inhibition of platelet thromboxane A2 may be the pharmacological basis for the effectiveness of aspirin in treating pregnancy-induced hypertension. Aspirin can virtually abolish thromboxane A2 production in patients receiving fish oil. Thus, aspirin has been and will be the standard reference compound for long-term oral treatment of platelet hyperactivity, most notably in the secondary prevention of myocardial infarction.