Heparin-induced thrombocytopenia (HIT) represents a prothrombotic adverse reaction to heparin caused by antibodies that target complexes of heparin and platelet factor 4 (PF4) (H/PF4). In some patients, this leads to platelet activation, aggregation and thrombosis (heparin-induced thrombocytopenia with thrombosis; sometimes abbreviated HITT). It is counterintuitive that a therapy (heparin) aimed at preventing or treating thrombosis may instead lead to thrombosis. Management of HIT is facilitated by early recognition and switching to a non-heparin anticoagulant. Diagnosis involves a clinical assessment of the probability of HIT, and in appropriate cases, immunological testing for anti-PF4 antibodies, and if possible, confirmation of pathological antibodies using functional platelet activation assays. There are a variety of anticoagulant options, in part dictated by geographical availability (including regulatory clearances). This chapter briefly describes the pathology of HIT and details its diagnosis and management.