Microorganisms mediate the cycling of sulfur compounds, and isotope fractionations are frequently involved in sulfur-compound metabolism. For example, large fractionations of up to 45 per mil are associated with dissimilatory sulfate reduction, and the disproportionation of sulfur compounds like elemental sulfur, sulfite, and thiosulfate also produce 34S-depleted sulfides. The magnitude of fractionations is sometimes dependent on environmental conditions like oxygen availability or sulfate concentrations. Therefore, the isotopic composition of sulfur compounds may reflect the type of organisms responsible for sulfur cycling, and the environment where they lived. This talk will outline the microbial processes responsible for isotope fractionation in nature, and the environmental factors influencing the magnitude of fractionation. Specific examples will demonstrate how these considerations address specific geological problems like the evolution of the sulfur cycle over Earth history, and the biological processes involved in sedimentary sulfur compound formation.