Heparin has been used extensively in the treatment of venous thrombosis and pulmonary embolus and more recently as a prophylactic measure in high-risk patient populations. The chemistry and biologic, pharmacologic, and physiologic actions of heparin are discussed, with some emphasis on the role of antithrombin III. The various modes of heparin administration, dosage regimens, requirements, and indications for therapy are reviewed. Full-dose and minidose heparin therapy are discussed in detail. The advantages and disadvantages of the various laboratory tests used to monitor heparin therapy are covered, along with standardization of heparin preparations. Finally, complications associated with heparin therapy are pointed out.