Heparin‐induced thrombocytopenia (HIT) is an antibody‐mediated adverse effect of heparin. It is highly prothrombotic and treatment usually requires substitution of heparin with a rapidly acting non‐heparin anticoagulant; vitamin K antagonists (warfarin) are contraindicated during the acute phase of HIT because their use can precipitate limb necrosis due to microthrombosis. Prophylactic platelet transfusions should be minimised. Given these special treatment considerations, the challenge is to distinguish HIT from non‐HIT thrombocytopenia. Management of HIT requires knowledge of immunohaematology and haemostasis. This chapter tabulates the features of HIT with particular relevance for the transfusion medicine specialist. Two general types of assays detect HIT antibodies: platelet activation (or functional) assays and PF4‐dependent immunoassays. Treatment of HIT should focus on rapidly acting, non‐heparin anticoagulants. There are two main classes of therapies: long‐acting indirect (antithrombin‐dependent) factor Xa inhibitors (danaparoid, fondaparinux) and direct thrombin inhibitors (argatroban, bivalirudin).