This book summarizes the numerous cases of fatigue damage of steel structures that have been reported over the past years, as well as the repair and retrofitting technologies that have been proposed and applied to solve the issue. It focuses on the causes and repair practices and provides comprehensive information to anyone with interest in the topic. This book was written as part of the activities of the International Institute of Welding (IIW), Commission XIII (Fatigue of Welded Components and Structures) Working Group 5 (Life Extension of Welded Structures by Repair, Retrofitting and Structural Monitoring; hereafter referred to as IIW XIII WG5). The author is the former chairman of IIW XIII WG5.