Low molecular weight heparins (LMWHs) are parenteral anticoagulants that are widely used for the prevention and treatment of thromboembolic disease. These agents possess several advantages compared to standard heparin, including a more predictable anticoagulant response, better bioavailability allowing for subcutaneous therapy, and a longer half-life. Laboratory monitoring of the LMWHs uses an anti-factor Xa assay, but monitoring is generally not necessary for most patients. However, certain patient populations do benefit from an individualized approach to therapy and, in some cases, therapeutic monitoring, because of an increased bleeding risk and/or relative contraindications to anticoagulant therapy. For example, since LMWHs are cleared by the kidney, they must be used with caution in renal insufficiency. LMWHs are safe and effective during pregnancy but may not be the optimal antithrombotic agent in pregnant women with prosthetic valves. In addition, management around the time of delivery can be difficult because of the bleeding risk, particularly associated with epidural anesthesia. Therapeutic monitoring also may be useful for the morbidly obese, in children, and for patients with malignancy.