This thesis presents the description, analysis and design of a high frequency parallel processing ac uninterruptible power supply. Compared to the standard UPS system used today, the proposed system has higher efficiency, smaller size and less weight, this is due primary to the usage of a high-frequency switching topology.  The main power circuit configuration is shown to describe the steady state behaviour of the system in both ac line supply and battery backup modes of operation. A stability analysis is developed to describe the system behaviour against the load variation and the input disturbance. The detailed design of the output voltage control loop is carried out. A transient analysis is presented to describe the transition between the two operation modes of ac line supply and dc battery backup. From this analysis a fast switching transition control circuit is designed. A design procedure is illustrated to select the system components for a 300W UPS. The corresponding system performance such as efficiency is evaluated. All the theoretical results obtained in this thesis are experimentally verified.