This chapter deals with the stabilizers for the chlorinated solvents, which were by far the most frequently used solvents for degreasing, dry cleaning, cold cleaning, and dozens of other industrial applications. 1,4-Dioxane, a probable human carcinogen, has manifested as the most significant drinking water and environmental contaminant among the dozens of solvent stabilizers used for chlorinated solvents. The chapter focuses on solvent stabilizers added to the four major chlorinated solvents: methyl chloroform, trichloroethylene, perchloroethylene, and dichloromethane. It explores the uses of solvents in different applications, and the corresponding requirements for solvent stabilization, in substantial detail. While the environmental engineer or hydrogeologist's primary focus may be fate and transport of the contaminant in the subsurface, and the contaminants' physicochemical properties governing design of remediation systems, understanding the genesis of contaminants pays dividends for understanding both fate and transport and remedial design.