Low molecular weight heparin (LMWH) is used in the treatment of various thrombotic disorders. The clinical indications, pharmacology, and complications of therapy are reviewed. The indications for the need for monitoring include renal failure, obesity, pregnancy, and pediatric use. Monitoring focuses on the anti-Xa properties of LMWH. The two major assay types, chronometric (clotting) and chromogenic (amidolytic), are reviewed and compared.