This introductory chapter presents the most relevant structural, physical, and chemical properties of clay minerals for the formation of nanocomposites with polymers. The general principles of silicates classification are outlined in order to better understand the structures of the various types of clay minerals as phyllosilicates. Cation exchange capacity (CEC), surface area, porosity, and rheological properties of clay minerals are briefly discussed. The physico-chemical properties of clay mineral layers, including the reactivity at the edges surfaces, are introduced together with their consequences for the various mechanisms of clay-polymer interactions. The chapter closes on a brief presentation of synthetic clay minerals and a general introduction to clay polymer nanocomposites.