The term "clay" was used in everyday language long before being imbued with a well-defined scientific meaning. Therefore, it is not surprising that it carries different connotations to different communities. To the industrialist, it is a raw material available in large amounts at cheap prices, characterized by its macroscopic properties relative to various applications. To the geologist working in the field, it is a particular secondary mineral largely found in weathered deposits from sedimentary or volcanic origin. To the chemist and mineralogist, it refers to a particular type of mineral structure defined at the atomic level.Recent recommendations of the JNC 1 advise to use the term "clay minerals" to refer to precisely determined crystallographic structures, and define "clays" in terms of macroscopic properties. 2 Therefore, a natural clay will consist of a/several clay minerals mixed with additional minerals as impurities. However, the distinction is not always clearly made and many papers that use well-defined clay minerals will refer to them as "clays" because the full denomination is somewhat cumbersome [1].Here we will build on the crystallographic view, which is the most rigorous, and try to indicate how the atomic structure dictates the properties at other levels.The most salient structural feature of clay minerals is that they are layered. That is to say they belong to a large class of inorganic compounds built by the stacking of Rubber-Clay Nanocomposites: Science, Technology, and Applications, First Edition. Edited by Maurizio Galimberti.