The structures of clay minerals have been known, in general terms, for upwards of half a century. Both mineralogists and soil scientists have been investigating the subject for their own purposes over at least as long a period, and a large literature, not necessarily well known to catalyst chemists, now exists. It is not the intention of this contribution to provide yet another account of the crystallographic and chemical properties of clay minerals at the research level: such is readily available, most recently, for example, in the book edited by Newman (ref. 1). The aim, rather, is to survey the common clay mineral types in a simple way and then to use this background to make some points relevant to the use of these minerals in the synthesis of pillar analogues. The key to understanding is not just the unit cell structure but what the consequences of it are in terms of surface properties like charge density and chemical group reactivity. Clay minerals are hydrous layer silicates of the so-called phyllosilicate family, in which the extremely stable SiO4 tetrahedral structural unit has polymerised to form two-dimensional sheets. This occurs through the sharing of three of the oxygens at the corners of the tetrahedra. The oxygens, which can be imagined as being at the base of an equilateral triangle (basal oxygens), form a repeating pattern of regular hexagons. This is illustrated in Fig. 2.1. The honeycomb pattern has the appearance of a graphite sheet, but with 0-Si-0 links instead of C-C. Diagrams such as those in Fig 2.