This book is primarily a textbook. It is written for engineers, students and teachers, and it should also be useful for people working on various topics related to fatigue of structures and materials. The book can be used for graduate and undergraduate courses and for short courses for people already working in the industry, laboratories, or research institutes. Furthermore, the book offers various comments which can be useful to research-workers in order to consider the practical relevance of laboratory investigations and to plan future research. An important theme of the book is the understanding of what happens in the material of a structure in service if the structure is subjected to a spectrum of cyclic loads. Knowledge of the fatigue mechanism in the material and how it can be affected by a large variety of practical conditions is essential for dealing with fatigue problems. The designer of a dynamically loaded structure must “design against fatigue”. This includes not only the overall concept of the structure with related safety and economic aspects, but also questions on detail design, joints, production and material surface quality. At the same time, the designer must try to predict the fatigue performance of the structure. This requires a knowledge of the various influencing factors, also because predictions on fatigue have their limitations and shortcomings. Similar considerations arise if fatigue problems occur after a long period in service when decisions must be made on remedial actions.