Landfill leachate is generated as a result of reactions between water percolating through the landfill and wastes. Under normal conditions leachate is found at the bottom of landfills and from there, its movement can be controlled with collection systems to be treated, discharged, or recirculated. Landfill leachate collection systems are positioned above the liner and are designed to collect liquid under gravitational flow for the entire active, closure, and post-closure periods. Clogging of any portion of the system can lead to higher hydraulic heads and increase the potential for leakage through the liner. To reduce the quantity of municipal solid wastes (MSW) requiring landfilling, many municipalities have adopted waste-to-energy (WTE) facilities that yield energy in the form of combustible gases and noncombustible residues. Disposal practices for WTE residuals include landfilling in monofills or co-disposal with MSW and other materials such as residues from water and wastewater treatment facilities. There has been concern about co-disposal practices, because the impacts on leachate quality and waste interactions are not well known yet. This research was conducted to evaluate clogging of leachate collection systems due to co-disposal of MSW and combustion residues from WTE facilities. The use of laboratory lysimeters in conjunction with batch tests to predict short-term and long-term leaching characteristics of noncombustible residues from WTE facilities was also evaluated. Laboratory lysimeters were used to simulate monofills (WTE residues and MSW) and co-disposal practices. Relationships between waste composition and leachate quality were evaluated over a seven month period.