The principles of transverse and longitudinal stability at rest and at speed are discussed together with means of determination. Unsteady longitudinal motions and porpoising are outlined with results from experimental investigations to identify stability boundaries. Theoretical analysis of motions in a seaway is introduced, followed by analysis of maneuverability. Forces including impact forces in waves are discussed, and the generation and suppression of spray, together with the influence of hull geometry on forces and motions. Drag increment, potential speed loss, and the issue of broaching in following waves are discussed. Seakeeping model tests and their use in the estimation of forces and accelerations are described, followed by theoretical calculations available to calibrate with testing. The influence of hull geometry on motions and accelerations is also discussed.