A monograph of this size could not pretend to be a comprehensive account of the defect solid state or its role in chemistry; it is designed to stimulate interest and research. Deviations from the fundamental laws of chemistry were first studied in detail by physicists; their implications were appreciated at that time by a handful of structural and inorganic chemists only and, until recently, all but ignored by the physical chemists. No doubt the concept is distasteful to those whose training in chemistry is classical, for it strikes at the foundations of order in inorganic chemistry and therefore introduces difficulties into its understanding and teaching; it threatens, however, to intrude into all phases of the chemistry of the solid state.