Achromatium is a genus of large unicellular sulfur bacteria. Despite being first described in the late 19th century, no Achromatium spp. have yet been isolated in culture, and for over 100 years, knowledge of their ecology, physiology and relationships to other bacteria has been scant. In recent years, the application of culture-independent techniques combined with in situ process measurements and single-cell activity measurements in sediments harbouring large Achromatium populations, has expanded our knowledge of these bacteria. Aspects of carbon and sulfur metabolism in Achromatium are now better understood, but their preferred electron acceptor(s) remain unknown. Unexpected diversity has been uncovered in Achromatium populations and it is now clear that the organism routinely described as Achromatium oxaliferum actually comprises several distinct Achromatium spp.