The control design of systems with lightly damped high frequency oscillatory modes is treated. Systems exhibiting these lightly damped modes include turbine generators and large flexible structures. Frequently the dynamics of these high-frequency modes are not known very accurately. As a result, the design objective is to improve the slow dynamics without destabilizing the high-frequency modes. From a classical frequency domain design viewpoint, this is in part a filtering problem requiring some kind of high-frequency filter. The singular perturbation method is used to systematically develop a two-time-scale method to design these high-frequency filters and provides guidelines on the kind of high-frequency filter to be used. In addition, the method provides an asymptotic error analysis. The design is illustrated with an example representative of a power system model with a two-mass turbine-generator model.