Lower limb robotic devices, like prostheses and orthosis, are required to work closely with human limbs, and thus an effective control framework and reliable design are both critical. This dissertation presents novel methods to control a DC powered knee prosthesis, a pneumatic prosthesis, and the progress of controlling a multifunctional orthosis. Moreover, this dissertation also presents a novel pneumatic knee prosthesis design. A novel high-level controller controls the DC powered knee prosthesis by utilizing the Electromyography (EMG) with biomechanical model. The controller combines an active control component that reflects the wearer's motion intention, with a reactive control component that implements the controllable impedance critical to the safe and stable interaction. The effectiveness of the proposed control approach is demonstrated through the experimental results for arbitrary free swing and level walking. A sliding mode low-level controller is applied to control the pneumatic prosthesis to overcome the highly nonlinearity from the properties of pneumatic muscle and the design of prosthesis. The effectiveness of the controller is demonstrated though experiments. The progress of making a complete control algorithm for a multifunctional orthosis consists of two major parts. One is the user movement classification methods. There are a total of three classifiers: the walk-to-stop classifier, the speed-changing classifier, and the movement start classifier, which includes climbing up a stair, climbing down a stair and level walking. The classification rate of all three qualifiers is 90% or more. The second major part of the research is high-level controllers for different functions. A high-level fuzzy impedance controller, which increases the flexibility of a regular impedance controller, has been developed for speed adaptive walking control. The effectiveness of the controller is demonstrated through simulation. A novel knee prosthesis design which utilizes the rope pulley mechanism and slider crank mechanism. In the pulley design for the rope pulley mechanism, a superellipse pulley is chosen to give more variation. The parameters in those mechanisms and the prosthesis are optimized, so that the knee torque from the prosthesis is close to that in a biological leg. The design also reserves space for the components of an ankle prosthesis.