Colourless sulfur bacteria were among the first groups of biogeochemically important bacteria to be studied by microbial ecologists. This was in part due to the fact that several species are large and often produce macroscopically visible structures such as mats. Although some of these sulfur bacteria have been studied for over a century, few have been obtained in axenic culture and thus, until recently, relatively little was known regarding their relationship to other bacteria or their role in the environment. The introduction of molecular biological methods to microbial ecology has allowed many of these distinctive uncultured bacteria to be characterized phylogenetically and in situ measurements have revealed aspects of their behaviour and their involvement in specific biogeochemical processes. Culture based studies of some morphologically distinctive sulfur bacteria, principally isolates of different Beggiatoa species, have revealed that they exhibit a wide range of metabolic activities that span heterotrophy to obligate chemolithautotrophy. This knowledge has informed studies of uncultured sulfur bacteria such as Thioploca and Achromatium. The combination of molecular biological analyses, chemical and process measurements, and culture based studies that have allowed the ecology of these bacteria to be interrogated are discussed.