This paper presents an overview of the design and control of a fully self-contained prosthesis, which is intended to improve the mobility of transfemoral amputees. A finite-state based impedance control approach, previously developed by the authors, is used for the control of the prosthesis during walking and standing. The prosthesis was tested on an unilateral amputee subject for over-ground walking. Prosthesis sensor data (joint angles and torques) acquired during level ground walking experiments at a self-selected cadence demonstrates the ability of the device to provide a functional gait similar to normal gait biomechanics. Battery measurements during level ground walking experiments show that the self-contained device provides over 4,500 strides (9.0 km of walking at a speed of 5.1 km/h) between battery charges.