Traditionally administered unfractionated heparin (UH) is a heterogeneous mixture of polysaccharide chains of varying length. Heparin is now available in new formulations, most notably the low-molecular-weight heparins (LMWHs), which possess pharmacology that is similar to but distinct from UH. Key advantages of LMWHs include improved bioavailability and longer half-life, more predictable anticoagulation that requires less laboratory monitoring, and fewer serious side effects. The article reviews the pharmacology of heparin and discusses the addition of LMWHs to the family of anticoagulant drugs.