Ankle foot orthoses (AFOs) can be used to ameliorate the impact of impairments to the lower limb neuromuscular motor system that affect gait. Existing AFO technologies include passive devices with fixed and articulated joints, semi-active devices that modulate damping at the joint and active devices that make use of a variety of technologies to produce power to move the foot. Emerging technologies provide a vision for fully powered, untethered AFOs. In this dissertation, a novel portable powered ankle-foot orthosis (PPAFO) cabable of providing untethered assistance during gait is presented. The PPAFO provides both plantarflexor and dorsiflexor torque assistance via a bi-directional pneumatic rotary actuator. The system uses a portable pneumatic power source (compressed CO2 bottle) and embedded electronics to control the motion of the foot. Experimental data from two impaired and five healthy subjects were collected to demonstrate design functionality. The impaired subjects had bilateral impairments to the lower legs that caused weakness to the plantarflexors, in one case, and to the dorsiflexors in the other. Data from the healthy walkers demonstrated the PPAFO’s capability to provide correctly timed plantarflexor and dorsiflexor assistance during gait. The results from the impaired subjects demonstrated the PPAFO’s ability to provide functional assistance during gait. Additionally, this dissertation presented a modeling and control approach to address limitations present in the PPAFO through the introduction of a new hardware configuration and new control architecture. A combined model consisting of both the PPAFO and the human foot and shank segments was first derived and validated. Next, the current and the new PPAFO system configurations were evaluated both in simulation and experimentally during three simplified functional gait tasks:​ (1) motion control of the foot at the start of the gait cycle, (2) plantarflexor torque assistance during late stance, and (3) dorsiflexor position control of the foot during swing. The resulting analysis showed that the new system configuration both outperformed and was more efficient than the current PPAFO configuration. The stringent design requirements of light weight, small size, high efficiency and low noise make the creation of daily wear assist devices challenging, but once such devices appear they will present new opportunities for clinical treatment of gait abnormalities.