The double hull structure has been applied to the bottom of cargo ships in general, and also to the newer types of ship such as container shipsContainer ship and LNGLNG carriers. Furthermore, it was regulated that a large tanker should be provided with the double hull structure for the bottom and the side shell early in 1990’s. The double hull structure of a cargo ship can prevent hold flooding when the outer shell is wrecked by groundingGrounding and collisionCollision. For a large container ship,Container ship the double side hull structure is to be applied to maintain torsional strength which is reduced by the large hatch openings. These double hull compartments are used as ballast tanks. The double side hull structure works to protect hazardous cargo or equipment, especially for the nuclear reactor in a nuclear ship or tanks for dangerous liquefied gases. In this way, there are many aims in applying the double hull structure, and the double hull structure has to be designed to meet these aims. In this chapter, discussions are made on the structural system of a double hull structure, a comparison with the single hull structure, and how to design the double hull structure of various kinds of vessels to meet the aims.