Design of a Hybrid Passive-Active Prosthesis for Above-Knee Amputees. This thesis presents the first energetically autonomous hydraulically powered prosthetic knee. The primary innovation of the knee is a hybrid design with two separate hydraulic modes of operation: an active mode driven by a pump, and a passive mode controlled by a variable position valve. The electrohydraulic design combines the safety, comfort, and versatility of a state of the art microprocessor controlled hydraulic knee with three new benefits. First, by pumping fluid into the hydraulic cylinder, the necessary knee angle during the swing phase of walking to clear obstacles in the path of the foot is achieved. Second, adding power at the knee reduces hip torque required to swing the leg forward. Third, the power added at the knee helps lift the amputee's body weight, allowing the step over step ascent of ramps and stairs. These benefits should reduce back and hip pain, and should improve mobility to afford the amputee a healthier life.