Acetylsalicylic acid, is one of the most effective antiplatelet agents. It effectively reduces the risk of thrombotic events across wide spectrum of patients with cardiovascular disease. However, the treatment failures are relatively common and significant number of patients don't benefit from aspirin therapy. In the last decade the term "aspirin resistance" has been used to describe several different phenomena. This article exposes the difficulties in defining aspirin resistance, discusses the mechanisms by which resistance may occur and deals with its clinical impact. It is necessary to standardize a definition of aspirin resistance, to develop reliable tests for it, and to assess the clinical utility of testing on patients outcomes (Tab. 3, Ref. 60).