This dissertation presents novel innovation within the field of powered prostheses with particular emphasis on the control of pneumatic actuators and the design and control of powered prostheses. For the first work presented in Chapter 3, a pressure estimation algorithm is developed for a force controlled pneumatic cylinder. The algorithm provides the required chamber pressure information via the measurement of the actuation force and by establishing an average pressure derived from the inflow-outflow balance of the actuator. For the force controller design, the dynamic model of the entire system is presented, and the standard sliding mode control approach is applied to obtain a robust control law. The pressure estimation algorithm and the corresponding robust control approach have been implemented on an experimental system, and the effectiveness demonstrated by the sinusoidal and square-wave force tracking. Chapter 4 presents the simultaneous position and stiffness control of a pneumatic artificial muscle actuation system. In order to develop such a controller, the full nonlinear dynamic model of the system is developed in the work. Given the dynamic model, a robust multi-input-multi-output (MIMO) sliding mode controller is implemented to control the system. The simultaneous position and stiffness controller is then implemented experimentally, and the effectiveness is demonstrated for sinusoidal tracking of both the position and stiffness. A modified design of the pneumatic artificial muscle in order to improve the force output and reduce the energy consumption of the muscle is presented in Chapter 5. Theoretical analysis of the force gains and the possible reduced energy consumption is given. The theoretical analysis of the improved force output and improved efficiency as well as the overall hypothesis of the work is then validated experimentally. Chapter 6 presents the design and control of a prosthetic elbow prototype using the modified pneumatic artificial muscle design presented in Chapter 5. The elbow was designed to provide adequate flexion torque of the elbow, while a set of rotary springs allow for extension of the joint. In order to control the prototype, the full nonlinear dynamic model of the system is developed. Given the model, a single-input-single-output robust sliding mode controller is implemented for the control of the system. The robust controller is then implemented experimentally and the effectiveness is demonstrated for a step input and sinusoidal tracking of the elbow joint angle. The final chapter of this dissertation presents the design of a powered transfemoral prosthetic leg. The design of both the powered ankle and knee joint incorporates a new actuation design for each of the joints. Each of the joint actuation systems are fully optimized to provide adequate torque for various modes of ambulation of the prosthetic leg. Finally, the detailed design of the prosthesis prototype is given in Chapter 7.