Hyaluronic acid (HA) is an unsulfated glycosaminoglycan that is a ubiquitious component of the extracellular matrix. This chapter first introduces the chemical structure, biophysical properties, and the biological context of HA. Next, the production of HA from vertebrate, bacterial sources, and chemoenzymatic sources is described, along with relevant analytical methods and standards. Then, methods for chemical modification of HA are described, in which HA is converted to a wide variety of biomaterials for clinical and research use. The medical applications of HA are then surveyed, including ophthalmic surgery, injections for osteoarthritis and dermal fillers, wound healing, and use in cell therapy and tissue engineering. The final section describes resources available to HA researchers and discusses the future of HA science.