C ontrol systems must fundamentally trade off performance with robustness to plant uncertainty. For example, increasing the controller gain is desirable for improving tracking and disturbance rejection, but only up to a point at which uncertainty in the plant gain can potentially render the closed-loop system unstable. Plant uncertainty arises from the inevitable discrepancy between the true plant and its model. Hence, controller design aims at achieving an acceptable tradeoff between the conflicting goals of tracking or regulation performance versus robustness to plant uncertainty.Gain and phase margins are used in loop-shaping controller design as a measure of robustness. For the loop transfer function L(s) = K(s)G(s), where K(s) is the controller and G(s) is the nominal plant model, which is assumed here to be stable, a large gain margin obtained with a small |L( jω)| means that the control system is robust to gain uncertainty. But this robustness typically comes at the cost of lower tracking accuracy. Thus, a loop-shaping design seeks an acceptable balance between gain and phase margins and tracking performance.For plant models with additive or multiplicative uncertainty, gain and phase margins do not adequately characterize the resulting stability or performance robustness of the feedback control system. For these types of uncertainty, a more detailed frequency domain analysis is needed to characterize the tradeoff between performance and robustness. In this article, only stable plants are considered, and a stable additive plant uncertainty model is used as shown in Figure 1. By stable system, we mean a system that is bounded-input, bounded-output stable or, equivalently, a system that is asymptotically stable and whose transfer function is proper. For closed-loop systems, stable means internally bounded-input, bounded-output stable. To facilitate the discussion, every system represented by a transfer function is assumed to be causal.The performance versus robustness tradeoff is an important aspect of the development of H ∞ control theory during the 1980s [1], [2]. One of the original motivations for H ∞ control theory is to minimize the magnitude of the weighted nominal sensitivity W e (s)S(s),