Heparin-induced thrombocytopenia is a threatening complication of heparin treatment. The physio-pathological mechanism is the production of antibodies, the most frequent target of which is the complex heparin-platelet factor 4. These antibodies may activate the coagulation and lead to venous or arterial thromboembolic manifestations. Clinical features as well as functional and immunological tests are used for the diagnosis. The treatment consists in discontinuing heparin administration and in setting up an alternative treatment for which two drugs are indicated in France: Orgaran and Refludan.