Clay has always played a major role in human life. Clay raw materials are used and their value recognized in many economic branches, agriculture, civil engineering and environmental studies. This is largely because of their wide-ranging properties, high resistance to atmospheric conditions, geochemical purity, easy access to their deposits near the earth's surface and low price.Clay minerals, the essential constituents of argillaceous rocks, can be classified in seven groups according, to their crystal structure and crystal chemistry. Clay raw materials are divided in the same way into seven groups. An eighth group covers clay ochres and pigments. Further classification is based on the purpose-made technological application. Some examples show relations between the crystal structure or crystal chemistry of the dominant phyllosilicate and the technological properties. The chapter “Utilization of clay raw materials in industry and other human activities” is the most extensive. It gives information on the application of clay raw materials or individual clay minerals: in the production of foods, feedstuffs, beverages, paper, rubber, plastics, artificial leather, protective coatings for interior and exterior use, pharmaceutics, cosmetics, paints, pencils, pastels, porcelain, electro-porcelain and other fine ceramics, coarse ceramics and sialon ceramics; in the foundries, various branches of the chemical, petroleum and cement industries, agriculture and forestry; in the preparation of agrochemicals and special fertilizers, lubricating oils and gels, lightweight ceramics and effective sorbents; in the manufacture of mineral wool, in briquetting and pelletizing processes; as ingredients in grinding and polishing pastes, in the insulations of dumps of various kinds of waste (including toxic and radioactive waste), in thermally, electrically, acoustically and chemically resistant insulations, and in filters for the treatment of industrial, agricultural and similar outflows.The earth sciences use the clay minerals in the earth crust: (1) as indicators of the environment during weathering, allothi- and authigenesis in the sediments and in the study of the source areas of the detrital supply; (2) as pH indicators and indicators of processes in micro- and mega-environments and of changes in the course of diagenesis and metamorphosis. Mineralogical, petrological, geological and geochemical investigations directed to clay minerals serve as one of the correlation methods, in the recognition of processes in the petroleum-bearing sediments, coal-bearing formations, origin of riverine, lacustrine, marine and oceanic sediments and in the climatic, geodynamic, paleogeographical, stratigraphic and weathering rate interpretations.