The H^∞ control theory is currently the most powerful method for robust control theory, and is useful as well as practical because a great amount of software related to computer-aided control system design is available. However, it has some disadvantages in that the H^∞ control system is a conservative one and cannot deal with robust performance. This is due to maximum singular values. Doyle proposed a structured singular value instead of a maximum singular value. This is called ∞ synthesis theory and actively deals with robust performance using D-K iteration. This paper is concerned with computeraided design of active vibration control systems based on the μ synthesis theory. First, the paradigm of the μ synthesis theory is described concerning μ, robust performance, and D-K iteration. Next, the relationships between the μ controller, robust performance, nominal performance, and robust stability are discussed for vibration control systems.