Bentonites are specific types of clay, commonly formed by weathering of volcanic ash. Bentonites can adsorb more water and other substances than other clays. Because of their unique properties, great abundance, and relatively low cost of exploitation and production, bentonites are used in numerous applications. Bentonites are indispensable for iron ore processing and casting of iron, oil drilling (drilling mud), refining vegetable oils, beverage production and even pharmaceutical applications. Furthermore, bentonites will soon be used to isolate highly radioactive waste from the surrounding environment. Bentonite formation starts with the alteration of volcanic ash or other materials containing siliceous glass. This publication provides a review of current knowledge of bentonite formation and a new systematic approach to understand the underlying physicochemical processes. Analysing bentonites and understanding the behaviour of bentonites in different technological processes remain challenging. Some aspects are not yet completely understood, such as the effect of particle size and morphology on the rheological properties of bentonite. The authors discuss available analytical methods, their possibilities and limitations with special reference to the relationships between the properties of a bentonite and its performance. This publication introduces (data) and discusses a set of 40 bentonite/clay samples from all over the world. It includes compositional and mineralogical data for these samples, making it a valuable reference on bentonite clays worldwide. Finally, the expert authors provide information on the wide range of bentonite applications in various fields, which has rarely been compiled in a single volume before. (Imprint 2024)