These lecture notes focus on the synthesis of robust controllers for feedback systems, in the presence of unstructured perturbations. It is assumed, as a prerequisite, that the reader is familiar with the basic linear system and automatic control concepts. In Part 1, interpolation theory is used to solve various single-input-single-output (SISO) robust control problems. While the interpolation approach is awkward for multivariable systems, it provides a very natural 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 2 is devoted to multivariable systems. Two approaches are outlined: the Hankle-norm approach and the two-Riccati-equation approach. Although only a limited number of results are proven, the MATLAB software is presented for numerical solution. The book is addressed to researchers, engineers and students who require an introduction to robust control theory for unstructured plant perturbations. The presence of examples and exercises at the end of many chapters allow the book to be used as an introductory text in robust control courses.