Heparin is one of the oldest biological medicines, and has an established place in the prevention and treatment of venous thrombosis. Low-molecular-weight heparins (LMWH) have been developed by several manufacturers and have advantages in terms of pharmacokinetics and convenience of administration. They have been shown to be at least as effective and safe as unfractionated heparin and have replaced the latter in many indications. In this article the chemistry, mechanisms of action, measurement of anticoagulant activities, and clinical status of heparin and LMWH are reviewed.