ABSTRACTThe City of Champaign, Illinois, began controlled landfilling of its municipal refuse as early as 1904. Since that time, the disposal of solid wastes by sanitary landfilling has developed from a desirable concept, seldom utilized, to a science, carefully implemented and regulated. Illinois is typical of an area of humid continental climate whose citizens and industries depend heavily on ground‐water supplies developed from both nearsurface and bedrock aquifers. Seasonal water‐table fluctuations may be large in some parts of the State, but ground water is seldom far below the surface. Special problems are often encountered in design of protective systems and development of monitoring programs for solid waste disposal sites in this area to safeguard valuable ground‐water resources. Critical decisions, often involving considerable time and expense to the disposal site operator, must frequently be made by regulatory agencies responsible for licensing and surveillance of solid waste disposal sites in such a hydrogeologic environment. These problems have been compounded by the fact that State regulatory agencies have only recently realized the importance of hiring hydrogeologists, soil scientists, chemists and engineers to effectively implement and enforce their environmental control programs relating to solid waste disposal.This paper summarizes the experience in Illinois of development of a rational program for protection of the State's ground and surface water from the indiscriminant disposal of solid wastes. Classes of solid waste sites recognized in the State and their position in terms of the hydro‐geologic environment are presented. Means of controlling landfill leachate, either by natural renovation in subsurface materials or by engineered collection and treatment are discussed. Examples are presented which illustrate the in‐field implementation of protective systems and monitoring devices for ground water in Illinois, which should be generally applicable to other areas of the country of similar geology an