Adequate control authority for a STOVL aircraft during very low speed and hover flights is critically important and must be considered during the aircraft preliminary design and refinement stages. Total control authority must be sufficient to trim the aircraft; to maneuver the aircraft in all axes in a satisfactory manner; to suppress external disturbances such as winds and gusts; to counter environmental changes such as atmospheric conditions, hot gas re-ingestions and suckdown effects; to counter asymmetric weapons loading; and to provide stability if automatic stability augmentation system is required. These considerations will impose additional requirements on the propulsion system and, if not dealt with early in the design process, may compromise the total performance of the aircraft. In this paper, the control authority requirements of a STOVL aircraft as imposed by the flying qualities requirements and other environmental considerations will be discussed. The effects of these requirements on the sizing of the reaction control system and the propulsion system will be illustrated by a design example. How control authority requirements may affect the overall aircraft sizing will also be illustrated.