Based on a series of H-I launches in Japan, a review of control system designs of expendable launch vehicles is presented in terms of their flexibility. The equations of motion are first formulated for lateral and longitudinal dynamics of a flexible vehicle, after which the aerodynamic loads caused by in-flight atmospheric wind are described in relation to trajectory planning. Included are mode analyses for launch vehicles and time responses to the flight environment. Finally, stability analyses for lateral and longitudinal vibrations of launch vehicles are presented with special emphasis on POGOs.