An important consideration in the design of many control systems is decoupling (each command input controlling only one output). The 2DOF (two-degree-of-freedom) or 3DOF (three-degree-of-freedom) structure is ideally suited to this task since only one of the degrees of freedom is affected by the decoupling requirement. The decoupling constraint on this degree of freedom, however, can lead to some deterioration in achievable system performance. A design procedure which minimizes this performance deterioration is established here. Moreover, the parameterization of the class of all decoupled systems is given for which the cost functional representing system performance remains finite.