The paper deals with the robust design of an MIMO feedback control system. It is divided into two parts. The first part presents a necessary and sufficient condition for the robustness of a unit feedback system. The uncertainty model considered in this analysis is nontrivial, and the present approach is applicable to nearly decoupled systems as well as nondecoupled systems. The second part of the paper presents an effective technique for the design of a robust, nearly decoupled MIMO system using the result of Part I. The design technique to be presented in Part II can be applied to a nonminimum phase plant.