In this paper, a generalized loop-shaping method is developed to deal with various uncertainty descriptions for robust control design. The method is based on the proposition that the singular values of a feedback system matrix can be shaped by the addition of a perturbation matrix on either the given initial controller or the nominal plant model. The applicability of the method to the case of structured perturbations is discussed. The block sensitivity of the M-Delta structure is exploited, yielding bounds on the structured singular value. These bounds allow us to construct an interactive design strategy for various controller modifications to shape the blocks of the M matrix that have the strongest impact on mu.