The most important thermal, chemical, physical, and biological methods for treating hazardous wastes and the fate of their land-disposed residues are reviewed and evaluated. Technologies are described as major, minor, and emerging according to their stage of development or application to hazardous waste; major ones include rotary kiln, liquid injection, and cement kiln incineration; neutralization, chemical oxidation-reduction, and ion exchange; filtration, distillation and settling techniques; and activated sludge, aerated lagoon, and landfarming treatment. Emerging technologies include molten salt and fluidized- bed combustion, liquid-ion extraction and other processes, none of which are considered to be outside the realm of current or future economic feasibility. In addition, waste reduction strategies and the land burial of stabilized/solidified wastes are discussed. Residues from these technologies vary widely according to waste type and composition, but a common component in many of them is heavy metals, which, as elements, cannot be further degraded to other products. The results of the available literature suggests that these metals will be retained in clay liners beneath a landfill through the mechanism of cation exchange, with the adsorption of metals favored by their smaller hydrated size, lower heat of hydration, and in some cases, higher valences than the naturally occurring alkali earth metals. Other important factors include ionic activity, the pH and ionic strength of the solution, the presence of complexing agents, and the possible surface heterogeneity of the clay. In soils, metal binding through cation exchange with clay is augmented by adsorption onto iron and manganese oxides and complexing with organic matter such as humic acids. Many field studies with landfarmed metal-bearing wastes show that these mechanisms are usually sufficient to retain metals to within several inches of their zone of application.