Plates make up the main hull structure such as shells, decks and bulkheads, in conjunction with secondary supporting members such as stiffeners and primary supporting members such as girders. The strength of such plates is most important since many kinds of load act directly on the plates, especially water pressure and longitudinal bending. The strength of plates is usually taken as a plane rectangular plate having uniform thickness. The boundaries, i.e., the four sides of a plate, are considered as fixed, simply supported, or elastically supported, in accordance with the kind of loads. Decisions relating to the boundary supporting conditions are very important because the calculated deformation and stress depend on these conditions. In this chapter, comprehensive discussions are made with respect to strength and practical design of plate members, including strength of plates under lateral loads, effect of simultaneously acting axial loads, buckling strength under in-plane loads, stress concentration and efficient opening shape, materials and roll direction, etc.