Heparin-induced thrombocytopenia (HIT) is a severe immunological adverse effect of heparin treatment. Recently the pathogenesis of HIT has been resolved regarding the mechanisms of platelet activation, the nature of the most important antigens and the involvement of the clotting cascade. HIT seems to be associated with massive generation of thrombin, which contributes to the thromboembolic complications. Based on these findings, treatment of patients with acute HIT should include cessation of all heparins and further treatment with an anticoagulant with antithrombin activity. Currently, the two most important compounds for further anticoagulation of HIT-patients are danaparoid-sodium and recombinant hirudin.