Heparin is a complex polysaccharide, consisting of repeating dissacharide subunits, which exerts its anticoagulant effect by potentiating the inhibition of activated clotting proteins by the naturally occurring inhibitor antithrombin III. The primary indication for its use is venous thromboembolic disease where an average 20,000 to 40,000 USP units are initially administered by constant infusion over 24 hours to prevent extension of an established thrombus. Subsequent treatment is based on the therapeutic response, usually monitored by a global clotting test such as the activated partial thromboplastin time (APTT). The chromogenic assay based on heparin-induced inhibition of activated factor X (Xa) is a particularly useful alternative monitoring test where thrombosis complicates pregnancy or is associated with the lupus anticoagulant. Subcutaneous heparin has also been used for the primary treatment of venous thrombosis but is more frequently used either for primary or secondary prophylaxis. Primary prophylaxis schedules usually employ a low dose (5000 units) administered 8- or 12-hourly without anticoagulant control, but a titrated subcutaneous heparin regimen has been successfully reported in elective hip surgery. Although arterial thrombosis is primarily initiated by platelet aggregates forming in vivo, heparin is commonly administered for acute arterial thromboembolism including peripheral arterial occlusion and repeated transient cerebral ischaemic events. The potential efficacy of heparin in disseminated intravascular consumption (DIC) remains to be firmly established, but is indicated where symptomatic thrombotic complications occur. Thrombocytopenia and haemorrhagic side effects may complicate heparin therapy, but bleeding complications may be minimised with the development of modified low molecular weight heparin which is currently undergoing clinical trial.