There comes a time ;n the development of each branch of science when the underlying principles gradually become clear to those working in that area. At such a juncture the availability of an appropriate monograph or textbook becomes particularly desirable and its lack tends to impede further progress. Solid state physics has passed through this stage during the last decade and has been well served by many definitive books at several levels. The closely-related chemical aspects of the defect solid state have not been so fortunate and, partly for this reason, this important area of chemistry has not received adequate attention in undergraduate and graduate courses. Indeed, unlike the molecular orbital treatment of covalent compounds and the Hgand field theory of transition element complexes, both of which have recently been well treated in numerous books at various levels of sophistication, the bonding theory of the defect solid state has been sadly neglected in student textbooks and monographs. The present book was written to remedy this situation and is based on lectures which the author has given at various times during the last 15 years.