Walking impairments are a common sequela of neurological injury, severely affecting the quality of life of both adults and children. Gait therapy is the traditional approach to ameliorate the problem by re-training the nervous system and there have been some attempts to mechanize such approach. In this paper, we present a novel impedance controller for the MIT-Skywalker. In contrast to previous approaches in mechanized gait therapy, the MIT-Skywalker does not impose a rigid kinematics pattern of normal gait on impaired walkers. Instead, it takes advantage of the concept of passive walkers and the natural dynamics of the lower extremity in order to deliver more "ecological" therapy. The proposed closed-loop control scheme can regulate the interaction between the walker and the treadmill and can provide the appropriate feedback to the walker during stance phase as well as at heel-strike and toe-off. Simulation results prove the feasibility of the impedance-based control scheme.