This master project aims at a continuation of the work around the Cheetah robot, a quadruped robot with passive compliant legs. The Cheetah’s legs are three-segmented pantograph legs featuring a linear spring. The running stability for different pantograph leg designs is investigated in a fully passive, energy conservative physical model. The effect of preloading the spring is examined as well. Furthermore a model for the pantograph leg whithout the restriction of energy conservation is presented. The model aims to achieve stable running by controlling the pantograph leg. The leg angle can be controlled by a motor at the hip in series with a compliance. Stable running is possible, if the system energy is held constant. For this purpose a strategy able to control the system energy by changing the leg length at lift-off is presented.