Fatigue is one of the primary reasons for the failure of structural components. The life of a fatigue crack has two parts, initiation and propagation. Dislocations play a major role in the fatigue crack initiation phase. It has been observed in laboratory testing that after a large number of loading cycles dislocations pile up and form structures called persistent slip bands (PSB). Fatigue crack growth is a crucial subject in the aircraft, railroad and other transportation industries as well as in the biomedical industry. This book presents the latest research in the field.