  The aim of this paper is in developing unifying frequency domain theory for
robust regulation of MIMO systems. The main theoretical results achieved are a
new formulation of the internal model principle, solvability conditions for the
robust regulation problem, and a parametrization of all robustly regulating
controllers. The main results are formulated with minimal assumptions and
without using coprime factorizations, thus guaranteeing their applicability
with a very general class of systems. In addition to theoretical results, the
design of robust controllers is addressed. The results are illustrated by two
examples involving a delay and a heat equation.
