This article outlines the controller design for a specific active transtibial prosthesis. The controller governs the power output of a DC motor attached to a four-bar mechanism and torsional spring. Active power reinforcement is used to assist the push off at later stages of the stance phase and achieve ground clearance during the swing phase. A two level control algorithm which includes a higher level finite state controller and lower level proportional-integral-derivative (PID) controllers is applied. To implement this control algorithm, a digital signal processor (DSP) control board was used to realize the higher level control and an off-the-shelf motor controller was used to realize the lower level PID control. Sensors were selected to provide the desired feedback. A dynamic simulation was performed to obtain the proper PID parameters which were then utilized in a bench test to verify the approach.