A formulation is presented, based upon algebraic systems theory, to deal with the problem of robustness under combined parametrical and/or structural changes in the plant. The key design objective is the maintenance of the dominant closed-loop poles close to their nominal values, while prescribing their placement due to the un model led dynamics to the subsets of the complex plane, which have small influence in the system response. The mathematical framework is the ring of polynomial matrices for input-output descriptions and its relationships with the rational transfer matrix fraction descriptions.