In situ systems to accelerate the stabilization of waste deposits have been presented as alternatives to containment, isolation or excavation as methods for remediation of uncontrolled waste sites. In situ applications involve three essential elements: selection of a chemical or biological agent (reactant) which can react with stabilize the waste, a method for delivery of th ereactant to the deposit and a method for recovery of the reaction products or mobilized waste. The most promising applications for in situ treatment methods are for spill or plume types of contamination, where the contaminants are relatively evenly distributed and preferably in liquid form. Delivery of reactants to solid, heterogeneous, low permeability deposits will be more difficult. In situ methods may find particular application when used in combination with other remedial measures, for example, removal of the source material and in situ treatment of th eplume. Four reactant categories have been examined: biodegradation, surfactant-assistant flushing, hydrolysis, and oxidation. Of these, biodegradation and surfactant-assisted flushing appear most promising as in situ treatment techniques. For any treatment technique, the potential toxicity of the applied reactant and any intermediate compounds or by-products must be carefully evaluated. Furthermore, the potential for undesirable reactions with other contaminants present must be studied (e.g., oxidation of phenol with hydrogen peroxide may also oxidize chromium (III) to the more toxic hexavalent chromium). Methods of delivery of reactants based upon gravity include surface flooding, ponding, surface spraying, ditching, and subsurface infiltration beds and galleries. Forced injection (pumping) may also be used. Permeability is an important consideration in selecting the delivery system. Gravity delivery methods require a permeability of the soil/waste medium in the range 10p-1s cm/sec to 10p-3s cm/sec (280 to 2.8 ft/day). Forced injection is most effective at a permeability in the range of 10p-1s cm/sec to 10p-4s cm/sec (280 to 0.28 ft/day); below this permeability limit a potential application of forced injection for reagent delivery coupled with electro-osmosis for recovery may exist. Additionally, gravity systems should be considered only when the waste deposit lies in the unsaturated zone and when the depth to the bottom of the deposit is less than 5 meters (16 feet). Otherwise, forced injection should be considered. Recovery systems using gravity include open ditching and buried drains, and pumped methods include wellpoint and deep well systems. Basically, the same limitations that apply to delivery systems are also true for recovery systems. Gravity-induced recovery works best when the water table is within 5 meters (16 ft) of the surface. For depths in the range of 0-8 meters (0-26 ft), well points can also be considered. Depths greater than the suction limit (about 8 meters or 26 ft in practice) will require the use of down-hole pumps for recovery.