This chapter describes fundamental principles and applicability of solidification/stabilization (S/S) processes. S/S processes, in addition to solidifying, produce nonhazardous or less hazardous wastes by physicochemical transformations. The chemical testing of a waste is a function of both intrinsic properties of the waste form and hydraulic and geochemical properties of the site. Leaching behavior directly determines the performance of S/S as a hazardous waste treatment for a particular process and waste. S/S systems can be broadly grouped under organic and inorganic based on chemicals used. The two most common types of organic processes are polymerization and thermoplastic. The most common S/S systems are cement-based, pozzolanic processes, thermoplastic, and organic polymer. In general, the applicability of S/S processes is based on: soil and site characteristics, and contaminant location and properties. The primary elements of an effective S/S design include: waste removal, untreated waste transportation/storage, reagent storage, mixing, and S/S waste transportation/replacement.