Clay minerals are almost ubiquitous in soil and rock and are among the most reactive silicates. They affect the engineering behavior of soil and rock both as materials of construction and as foundation materials. In the petroleum industry, clay affects the permeability of reservoir formations, it is a common cap-rock, and it is also a constituent of the fluids used in drilling operations. Engineering behavior almost always involves clay-water interaction and this depends on the nature of water and solutions and on the composition and microstructure of the clay. In foundation engineering its role in soil-moisture interaction is illustrated by reference to problems resulting from the geological history of some North American soils and from engineering activities. In building materials, reference is made to its effect on concrete durability in aggregate-related problems. The importance of clay in petroleum engineering refers to authigenic clays in reservoir rocks, to clay behavior in the Alberta oil sands and to the use of clay minerals as a geothermal thermometer.--Modified journal abstract.