Hull girder vibration is treated in this chapter using mathematical methods (differential equation and energy approach). In the first part elementary vibration theory is presented, progressing from the SDOF system to the undamped vibration of the Timoshenko beam. The energy approach to vibration is presented next. In the next part ship vibration is presented. The types of vibration encountered in ships are discussed and classified, following which the distinguishing features of ship vibration compared to that of a uniform beam are presented. These relate to structural layout, design and operational aspects and the marine environment (added mass effect). In the next section vibration arising from steady-state excitation is described. This concerns vertical, horizontal and torsional vibration. Expressions for natural frequencies in each mode are given. In the case of vertical vibration the differential equations of vibration of a ship hull girder are obtained and expressions for natural frequency included in various publications compared. The differential equations of coupled vertical and horizontal vibration are obtained and springing is discussed. Vibration arising from transient loading is discussed and includes slam-induced whipping and whipping induced by bow flare impact.