This chapter presents the design and preliminary evaluation of a user-adaptive ankle foot orthosis (AFO). To begin with, according to structural characteristics of the human ankle as well as foot dimensions of an able-bodied subject, the ankle orthotic device is conceived. Then, based on a common two-degree-of-freedom (DOF) foot model, a coupled AFO–human system is setup. Further, the system's DOFs are derived; the device's mechanism of user adaptation is analyzed and verified using adams software. After that, the layout of a portable orthotic system, as well as a smart insole that detects gait phases, is illustrated. Finally, the orthotic system is tested on the aforementioned subject. Results show that, during the AFO-based walking with assistive torque, the foot's plantarflexion level before the swing stage and dorsiflexion level in the swing stage approximately increase by 3 deg and 4 deg, respectively, relative to the condition of AFO-based walking without assistive torque. Therefore, the orthosis has the potential to aid body propulsion and control toe clearance.