The hindgut microbiota of termites includes an abundant and morphologically diverse population of spirochetes. However, our understanding of these symbionts has remained meager since their first observation in termite guts by Leidy over a century ago, in part because none had ever been isolated in culture. Recently, this situation has changed dramatically with the application of cultivation-independent molecular methods to determine their phylogeny, and with the isolation of the first pure cultures. The emerging picture is that earth's termites constitute an enormous reservoir of novel spirochetes, which possess metabolic properties (H₂/CO₂-acetogenesis and N₂ fixation) hitherto unrecognized in spirochetes and which contribute to the carbon, nitrogen and energy requirements of their termite host. These discoveries help to explain the enigmatic dominance of CO₂-reductive acetogenesis over methanogenesis in the hindgut of many termites, as well as the old observation that elimination of spirochetes from the gut results in decreased termite survival.