Recent research in dynamic legged locomotion has indicated that the running dynamics of the robot are largely determined by the passive compliance of the running limbs. Many walking robot have variable stiffness limbs, the stiffness of which is tuned using mechanical actuators. Due to dynamic constraints, running robots cannot use the bulky variable stiffness mechanisms that can be implemented of walking robots. Standard legs for dynamic running robots have a single compliance which is determined when they are fabricated. This limits the adaptability of the robot, and creates a small range of operating conditions in which the robot can operates effectively. The need for variable stiffness legs has led to the development of a mechanically variable stiffness leg capable of a 200% change in stiffness. We developed a materials based variable stiffness leg and control system. A shape memory polymer resin was synthesized, and methods for shape memory polymer composite fabrication were developed. The composite materials were characterized, and a temperature control circuit was designed and fabricated. We present a new design for a variable stiffness leg which uses a shape memory polymer segment to acheive a 350% change in stiffness without any moving parts.