A serial elastic actuator (SEA) includes a physical compliance. Thereby the actuator provides the following characteristics: sensible for external mechanical impacts, decoupling loads from the motor side, low-pass filter behavior for loads and it stores energy. This characteristics privilege the actuator for applications with a direct human-robot contact like industrial, rehabilitation or teleoperation scenarios and for walking robots. The talk gives an overview of common applications and designs. It gives a more detailed view on SEAdevelopment at the DFKI project ’Capio’. Generally the implementation of a compliance cause hysteresis problems by friction or material characteristics. Depending of the positioning of elasticity it complicates the design development and increases the actuator size. To solve these problems a customized torsional disc spring is implemented in a serial elastic actuator module. The iterative spring design workflow from the CAD model to the large displacement FEM simulation is described. The DFKI design of a serial elastic module for a 5 Nm actuator is shown beside other research designs.