The Institute of Flight Mechanics of the DLR has developed a helicopter in-flight simulator, called ATTHeS (Advanced Technology Testing Helicopter System). This in-flight simulator is used for control law design, handling qualities investigations and, in an operational mode, for test pilot training. An explicit model-following control system has been developed which meets the demands of high bandwidth and high flexibility. The essential steps, to obtain a satisfactory status of the control system are described in this paper. The first step was to define an appropriate mathematical model of the testbed BO 105, which includes rotor degrees of freedom to reduce time delays, as these are normally used to describe model deficiencies. The second step was to conduct flight tests with the testbed and then determine the parameters of the defined model by system identification. The third step was the inversion of the above-defined and identified model, and building up the tools for the calculation of the feedforward part of the model-following control system. In a fourth step the design and calculation of the feedback structure was conducted. This was done by intensive simulation runs and a final refinement in flight tests. The fifth and final step was to conduct many flight tests for the evaluation of the performance of the overall system, to make the in-flight simulator available for operational investigations.