This chapter addresses the challenge of optimizing the parameters of membership functions in fuzzy-model-based controllers, particularly under conditions of imperfect premise matching. A novel frequency-domain algorithm is proposed, which establishes an explicit relationship between membership function parameters and the desired control performance. This development facilitates a systematic optimization strategy grounded in the nonlinear characteristic output spectrum (nCOS) function. In contrast to conventional search-based optimization techniques, the proposed method offers a more efficient and insightful approach, delivering comprehensive results within a significantly reduced computational timeframe. Moreover, it provides a deeper understanding of the underlying nonlinear dynamics, extending beyond the pursuit of optimal performance alone. The application of this method leads to substantial improvements in the effectiveness of fuzzy-model-based controllers.