ALEX is a series of exoskeletons designed for gait training. One of the fundamental features of ALEX is the ability to control forces at the foot in response to errors in the trajectory of the foot. A variety of studies have been performed with ALEX to show the feasibility of gait training of human subjects using a combination of force feedback, visual feedback, and audio feedback. ALEX II is a one-leg exoskeleton for gait training of stroke patients. ALEX III is a bilateral exoskeleton. A series of human studies have been performed with these exoskeletons to demonstrate the effectiveness of gait retraining.