Book excerpt: Sulfur is the tenth most abundant element in the universe and the sixth most abundant element in microbial biomass. By virtue of its chemical properties, particularly the wide range of stable redox states, sulfur plays a critical role in central biochemistry as a structural element, redox center, and carbon carrier. In addition, redox reactions involving reduced and oxidized inorganic sulfur compounds can be utilized by microbes for the generation and conservation of biochemical energy. Microbial transformation of both inorganic and organic sulfur compounds has had a profound effect on the properties of the biosphere and continues to affect geochemistry today. For these reasons, we present here a collection of articles from the leading edge of the field of sulfur microbiology, focusing on reactions and compounds of geochemical significance.