for the computation of the elastic states of dislocation loops by means of straight dislocation data. Chapter III presents the main results obtained so far in describing non-linear effects in the elastic field of straight dislocations, as well as in the study of the core configuration of dislocations by using semidiscrete methods. Chapter IV is devoted to the linear and non-linear theory of continuous distri- butions of dislocations and to its application to investigating the influence of dislo- cations on crystal density and on the low-temperature thermal conductivity of crystals. Chapter V deals with the modelling of point defects as rigid or elastic inclusions l in an elastic matrix, or as force !nultipoles. Finally, some of the results Q)ailable on the interactions between point defects and other crystal defects are briefly reviewed. Although the material in the text covers mainly the mathematical theory of crystal defects, the author has been constantly concerned with emphasizing the phYSical significance of the results and some of their possible applications. The reader can easily enlarge his information in these directions by reference to the stan- dard books on crystal defects by Cottrell {84J, Read {275J, Friedel [124J, Kroner [190J, van Bueren {365J, Indenbom {167J, Nabarro {258J, Hirth and Lathe [162J, or to the review articles by Seeger [286 J, Eshelby {Ill J, de Wit [385 J, and Bullough [50].