The biggest difference between fatigue assessment for existing structures and fatigue design of new structures is that the target structure exists, the condition of the structure can be directly observed, and the stresses that caused fatigue and the displacement behavior that caused its occurrence can be known. Fatigue assessment is performed because fatigue damage has occurred or is likely to occur, and the most important thing in fatigue assessment is to identify the cause of fatigue damage. If the cause is not identified, it is not possible to show the retrofit method. In this chapter, the flow of assessment of fatigue safety of existing bridges will be outlined. Stress measurements at potential fatigue locations, evaluation of the fatigue damage, and actual examples will be presented.