This book reviews problems in the mechanical behaviour of cyclically loaded metallic materials, primarily with regard to the nature of the fatigue process. The first edition of the book appeared in 1980. The present second edition represents a revised form of the original book and also covers recent developments in the field. As the book focuses on physical-metallurgical aspects, it occupies a unique and important position in the technical literature, which has so far been devoted mainly to engineering metal fatigue problems and their technical solution in specific practical cases. The book provides a compact review of current knowledge on physical metallurgical processes that accompany and affect the fatigue of metallic materials, and also presents the background for applying the new results to practical designing and to the selection of materials in engineering practice. The authors present an updated review of results from countries both in the east and the west and cover a relatively large field in a concise manner.