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A number of passive orthoses have been developed to provide gait assistance and rehabilitation for individuals who have lost the ability to walk. However, the required metabolic cost for legged mobility with conventional orthoses is huge, preventing its daily use. Using the forces/torques generated by external actuators is one of the effective ways to solve the problem induced from the considerable effort required for orthotic gait. For that, design of a compact, efficient and light weight actuation system and its delicate control are required. In Chapter 1, Powered Reciprocating Hip Orthosis, a novel hip actuator making use of the coupling hip mechanism of the reciprocating gait orthosis (RGO) is proposed. The RGO, a wearable and passive orthosis, provides paralyzed patients with hands-free standing and the ability for dynamic ambulation without any external actuation. Therefore, the mechanism of the RGO can be utilized effectively for improving the hip actuator design for a powered lower limb orthosis. Starting with this motivation, a powered limb orthosis combined with the conventional RGO was designed and its control structure was implemented on a paraplegic subject (T12 complete). First, a dynamic analysis is presented to identify and better understand the potential use of the RGO mechanism. For the analysis, the simple RGO dynamic model at a single support phase is obtained and its equations of motion are derived using Lagrange's equations of motion. Through the physical interpretation provided by the inverse dynamics, it is proved that the required maximum hip torque for stance phase is significantly decreased when the RGO hip coupling mechanism is applied. An unproved torso stability provided by the design is also investigated. Subsequently, overall control structure with a user-interface module is introduced to provide basic functions to the powered orthosis for locomotion. While implementing the designed orthosis with the human subject, a difficult problem on the dorsiflexion-activated passive knee orthosis of the conventional RGO is discussed and leads to Chapter 2: Friction Damping Control Knee Orthosis. A simple, but effective, microprocessor orthotic knee control method is presented with a novel knee joint design and an inertial measurement unit (IMU) sensor in Chapter 2. First, an electric hardware and a control structure with the IMU sensor are introduced. Next, to understand the knee dynamics and determine a control strategy, a lower extremity model is set up. Using this model, a dynamic analysis for ballistic walking with overshooting and circumduction of the hip joint motion, and downstairs walking is executed with the experimental hip motion data. Based on the observations for human hip joint motion and the obtained dynamic simulation results, a friction damping control method is proposed. Its implementation enables natural walking on level ground and provides the appropriate resistance of the knee joint in downstairs walking for assistance and safety to normal human subjects using the designed knee orthosis.