If structural damage to a ship occurs, it means that the load acting on the structure must have exceeded the maximum limit of structural strength. The hull structure has been planned originally by the designer in such a way that the structure can effectively resist the presumed maximum load, which is estimated from previous experience of failure modes of damage. Therefore, once damage occurs in the structure, it indicates that the magnitude of the load causing the failure is greater than that expected and/or the actual failure takes place in a different mode from the presumed one. By using that damage experience for planning the next structure, the designer is able to improve the accuracy of the load estimation and to accumulate knowledge relating to various modes of failure, and consequently the designer can achieve a design where there would be no future damage to the structure. When the designer starts a hull structure design, the very first step is to establish the magnitude, direction, probability of occurrence of the load, and also the structural failure modes experienced in the past. In this chapter, several failure modes, such as yielding, buckling, fatigue and vibration are explained.