This monograph presents the fundamentals of the mineralogy and geology of clays. The chemical or isotopic composition and the crystalline structure of clays is based on some relatively simple laws. Abundant illustrations help to understand the subject and can be used in courses. The organization of the book follows a logical structure. From the representation of the structure of minerals, the determination of their crystalline state by using X-ray diffractometer measurements (and other techniques) the thermodynamic and kinematic conditions of their formation is reconstructed. Then the genesis and alteration of the clays at their natural occurrences is discussed. In the last chapter it is stressed that clay minerals can form in variety of different environments: meteorites, lavas, subduction zones for example. The book is aimed for graduate students and scientists (geologists, geographers, engineers, environmental and soil scientists) who want to get a comprehensive introduction in the topic.