The properties of materials at high temperature play a vital role in their processing and practical use. The real properties of materials at elevated temperatures are very often governed by defects in their structure. Lattice defects may consist of point defects like vacancies, interstitial atoms or substituted atoms. These classes are discussed in general and specifically for oxides, nitrides, carbides and sulfides. Defect aggregates, shear structures and adaptive structures are also described. Special attention is paid to hydrogen defects which seem to play an important role in several materials.Defects in solids lead to transport properties such as diffusion and conductivity. These themes are thoroughly treated in this book, with examples from various materials being provided. Special attention is paid to the transport properties of grain boundaries. In high temperature corrosion and other types of oxidation, the diffusion of atoms through the reaction products is often the rate limiting step of the reaction. This book takes the reader from the theoretical treatment of defects to applications in high temperature corrosion. Reactions between metals and pure oxygen lead to the formation of oxides on the surface, and the reaction rates may often be related to the diffusion coefficients of the oxide. However, in practical use alloys are subjected to other severe gaseous atmospheres which may often lead to accelerated attack on the material. The severest condition, namely where a salt deposit is combined with oxidizing gases, is called hot corrosion. This and other types of corrosion are also covered. Finally, a chapter is devoted to the prevention of corrosive attack on materials by the addition of rare earth metals.The book has been published in honour of Professor Per Kofstad on the occasion of his 60th birthday. Professor Kofstad has for many years been active in the field of high temperature chemistry in all its aspects, from basic and theoretical work to its application in high temperature corrosion of metals and alloys. The various chapters have been contributed by his friends and colleagues, all of whom are international experts in the field.