Flue Gas Desulfurization (FGD) residue is produced when SOx and NOx are removed from gases produced during the combustion of coal. This by-product is becoming increasingly significant with more stringent air quality standards, and increasing fossil fuel consumption. FGD residues vary widely in their composition; ranging from products composed mainly of gypsum to materials which contain large quantities of fly ash. Factors that influence the final composition of the material include the source of the parent coal, and the type of FGD system used. With more advanced FGD systems sulfur removal is more efficient, and the material is of a higher purity. Older techniques frequently produce materials that are contaminated with fly ash. FGD residue does not contain significant quantities of heavy metals; contamination or mixing with fly ash contributes trace elements such as boron, arsenic and selenium to the material making their disposal a potential environmental risk. While many construction-based re-use avenues are being explored, the potential use of FGD as a soil amendment remains the most promising. Gypsum-containing materials have been used in the amelioration of physical soil problems such as water holding capacity and subsurface acidity. The production, reuse and environmental effects of FGD are discussed in detail in this chapter. A greenhouse-based pot study and a large scale mesocosm experiment indicate that while concerns about heavy metal leaching are not without basis, concentrations of B, Se and As do not exceed drinking water standards, and do not accumulate significantly in any of the plant species tested. Furthermore, FGD addition produced an increase in the dry matter production of cotton, corn, soybean and radish crops. Strict observance of safe upper limits for FGD application (approximately 100 t ha−1 in this study), and careful consideration of the site topography are necessary for the safe use of FGD material, and should follow careful characterization of the material for the extent of fly ash contamination.