The physicochemical properties of smectite clay minerals that determine their industrial utilization are reviewed. Smectite is the name used for a group of phyllosilicate mineral species, the most important of which are montmorillonite, beidellite, nontronite, saponite and hectorite. These and several other less common species are differentiated by variations in chemical composition involving substitutions of Al for Si in tetrahedral cation sites and Al, Fe, Mg and Li in octahedral cation sites. Smectite clays have a variable net negative charge, which is balanced by Na, Ca, Mg and, or, H adsorbed externally on interlamellar surfaces. The structure, chemical composition, exchangeable ion type and small crystal size of smectite clays are responsible for several unique properties, including a large chemically active surface area, a high cation exchange capacity, interlamellar surfaces having unusual hydration characteristics, and sometimes the ability to modify strongly the flow behaviour of liquids. In terms of major industrial and chemical uses, natural smectite clays can be divided into three categories, Na smectites, Ca-Mg smectites and Fuller's or acid earths. Large volumes of Na smectites and Na-exchanged Ca-Mg smectites and Fuller's earth are directly used in the foundry, oil well drilling, wine, and iron ore and feed pelletizing industries, and are also used in civil engineering to impede water movement. Significant volumes of Na smectites are used for various purposes in the manufacturing of many industrial, chemical and consumer products. Large quantities of Ca-Mg smectites are used directly in iron foundries, in agricultural industries and for filtering and decolorizing various types of oils. A significant fraction of the Ca-Mg smectites used for decolorizing has been acid treated. Large volumes of Fuller's or acid earths are commercially used for preparing animal litter trays and oil and grease absorbents, as carriers for insecticides, and for decolorizing of oils and fats. Natural Na smectites occur in commercial quantities in only a few places, but Ca-Mg smectite and Fuller's earth deposits of considerable size occur on almost every continent.