Seepage of leachate or the migration of noxious gases from landfill sites can pollute the ground and groundwater to a considerable distance from the source of the problem. Current trends and legislation dictate that future practice at waste disposal sites will be to provide a lining capable of eliminating or minimizing the migration of contaminants. Geotechnical considerations in the design and construction of low-permeability clay linings are addressed and laboratory test results for a range of clay types are presented in support of the arguments. The use of the moisture condition value (MCV) test in the selection of acceptable materials and the control of earthworks operations is discussed. A permeability requirement of no greater than 10-9 m/s for the clay lining dictates the upper limit to the acceptable MCV range while the shear strength dictates the lower limit.