Lower limb prostheses have traditionally been passive devices that lack the ability to generate net power at the joints. This absence of net power generation impairs the ability of passive prosthesis to restore biomechanically healthy function to lower limb amputees. Recent advances in battery, motor, and microelectronics technologies have enabled the possibility of powered (i.e., active) lower limb prostheses. Instilling a lower limb prosthesis with power, however, changes greatly the nature and significance of the prosthesis control and interface problem (i.e., a passive prosthesis can fundamentally only react to the user's input, but a powered prosthesis can both act as well as react). This talk describes the development of a lower limb prosthesis with a powered knee and ankle joint, and describes the control methodology through which the prostheses interacts with the user. Results are presented that indicate the effectiveness of the prosthesis and control interface.