MABEL is a bipedal testbed at the University of Michigan that was constructed in collaboration with Jonathan Hurst. The robot is planar, with a torso, two legs with revolute knees, and four actuators. Two of its actuators are in series with large springs for the purpose of enhancing both energy efficiency and agility of locomotion. The actuators are housed in the torso and the legs are light, placing the center of mass of the robot significantly above the hips. The presentation will focus on the development of a time-invariant feedback controller that respects the natural compliance of the open-loop system and realizes exponentially stable walking gaits. Experiments are presented that highlight different aspects of MABEL and the feedback design method, ranging from basic elements such as stable walking, to energy efficiency, fast walking, and walking over uneven terrain.