Clays are distinctive in at least two properties: plasticity and composition. Wetted clay deforms by application of relatively slight pressure retaining the deformed shape after release of the pressure. Clays are predominantly composed of hydrous phyllosillicates, called clay minerals, which exist in layers within the clay particles. The various minerals differ with regard to structure of the alternating layers, water content, and associated cations. The crystalline and paracrystalline groups of clay are presented as is the reliance of clay properties on composition and particle size. The surface area of the clay particles, often colloidal in size and usually platelike in shape, often dictates clay properties. The origin and locations of the various clays are discussed. Clay mineralogy is presented.