During recent years physicists in n1any countries have made rapid and important advances in the field of solid state physics. Semi-conducting materials, in particular, have become a subject of great interest by reason of their numerous practical applications and because of the fundamental insight into electronic and ionic processes resulting from these studies. The Reading Conference (10 to 15 July 1950), of which this voJ:.rme con- stitutes the Proceedings, provided research workers from this country and abroad with an opportunity for discussion of the most recent progress. Over 200 physicists and engineers took part, among whom it was a pleasure to welcome 80 visitors from other countries. The Conference was made possible by the financiaJ support of the International Union of Pure and Applied Physics (assisted by UNESCO) and the cooperation of the. Royal Society. Grateful acknowledgement is made to these organizations for the help received. Thanks are also due to Professor H. A. Kramers, Director, and Professor P. Fleury, Secretary General of the International Union, for much valuable advice during the preparations and the Editor wishes to thank Drs J. Friedel and C. A. Hogarth for their kind assistance.