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, that the reader is familiar with the basic linear system and au- tomatic control concepts. In part I interpolation theory is used to solve various single-input-single-output (SISO) ro- bust control problems. While the interpolation approach is awkward for multivariable systems, it provides a very natu- ral and simple approach for SISO systems. In particular the interpolation approach requires only elementary knowledge of complex variables, and provides a great deal of physical insight into various robust control problems. The required interpolation theory is developed in some detail. Part II is 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 results are 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 plant perturbations. The organization of the book as lecture notes and the presence of examples and of exercises at the end of many chapters allow to use the book as an introductory text in Robust Control courses.