These lecture notes focus on the synthesis of robust con-trollers for feedback systems, in the presence of unstruc-tured perturbations. It is assumed, as a prerequisites, thatthe reader is familiar with the basic linear system and au-tomatic control concepts. In part I interpolation theory isused to solve various single-input-single-output (SISO) ro-bust control problems. While the interpolation approach isawkward for multivariable systems, it provides a very natu-ral and simple approach for SISO systems. In particular theinterpolation approach requires only elementary knowledgeof complex variables, and provides a great deal of physicalinsight into various robust control problems. The requiredinterpolation theory is developed in some detail. Part IIis devoted to multivariable systems. Two approaches are out-lined: the Hankle-norm approach and the two-Riccati-equa-tion approach. In this part only a limited number of resultsare proven. However MATLAB software is presented for nu-merical solution. The book is addressed to researchers,practicing engineers, and students who wish to get an intro-duction to robust control theory for unstructured plantperturbations. The organization of the book as lecture notesand the presence of examples and of exercises at the endof many chapters allow to use the book as an introductorytext in Robust Control courses.