Bentonites have been used for several purposes, due to their excellent physical, chemical, and structural properties. The main applications of bentonites in Brazil are, among others, as agglomerants for foundry molds, in iron ore pelletizing, as drilling mud in oil and water wells, as wine clarifiers, in catalysis, at the chemical and pharmaceutical industries, for pollutants removal in water treatment plants, and in cosmetics. Since the 1980’s, bentonites have been used as barriers in landfills, thanks to the attractive characteristics of the montmorillonite, the principal clay mineral in the smectitic clays (popularly called bentonites). The inadequate accumulation of residues in soils and waters is identified as one of the most serious current problems. Nowadays, geosynthetic clay liners, or GCL’s, have been used as a geo-synthetic hydraulic barrier. They are constituted by a sodium or calcium bentonite layer encapsulated in geo-textiles or attached to geo-membranes, which are secured together by physical or chemical methods. This paper discusses GCL’s, their uses, the influences of the type of clay, the working principles, the related aspects to landfills projects, and also the main Brazil’s bentonite exploitation areas.