This article reviews the natural history, taxonomy, physical structure, growth requirements, and molecular structure of Borrelia burgdorferi sensu lato, the causative agent of Lyme disease. These spirochetal bacteria are maintained in nature through an infectious cycle between wild mammals and ticks. Borreliae are fastidious, slow-growing bacteria, found only in association with their arthropod or mammalian hosts in nature, and propagatable in the laboratory in a rich growth medium. The characteristic shape of borreliae is imposed by periplasmic flagella, located beneath the outer membrane and attached to the protoplasmic cylinder. The outer membrane of borreliae contains a number of abundant lipoproteins that are of serodiagnostic utility and currently under consideration as vaccine targets. The borrelial genome is unique in structure, organization, and copy number. Recent experiments demonstrate the feasibility of specific gene inactivation as a means with which to study the biology of borreliae and the pathogenesis of Lyme disease.