This chapter explains modeling and analysis of active filter systems using the generalized state space averaging method. It discusses digital control of active filters and series–parallel uninterruptible power supplies. The chapter presents a full digital deadbeat controller for active filters and uninterruptible power supply. It describes the deadbeat control of a simplified single-phase shunt active filter. Since its inception, very few studies have been done on modeling and analysis of active filters. In fact, most of the works have been on modeling of the control systems and operating principles of active filters. Some works have been done on modeling of electrical systems based on the switching functions method and the generalized state space averaging method. This research has shown the ability of the methods for modeling and analysis of power electronic circuits. In addition, simulation and experimental results are presented to help understand the theoretical description of the system.