Advantages and limitations of washout filters in feedback control of both continuous-time and discrete-time systems are discussed and generalizations that alleviate the limitations are presented. Some previously unpublished results in the Ph.D. dissertation of one of the authors (Lee, 1991) are presented in the context of their relation to the generalized results and to recent publications on delayed feedback control. We show that delayed feedback control (for discrete-time systems) extensively used in control of chaos is a special case of washout filter-aided feedback. Moreover, the limitations of delayed feedback control can be overcome by the use of washout filter-aided feedback, which gives rise to the possibility of stabilizing a much larger class of systems.