This book deals with the issue of fundamental limitations in filtering and control system design. This issue lies at the very heart of feedback theory since it reveals what is achievable, and conversely what is not achievable, in feedback systems. Recent developments in the field of fundamental design limitations include extensions to multivariable linear systems, sampled-data systems, and nonlinear systems. At this point in time, a considerable body of knowledge has been assembled on the topic of fundamental design limitations in feedback systems. It is thus timely to summarize the key developments in a modern and comprehensive text. This has been our principal objective in writing this book. We aim to cover all necessary background and to give new succinct treatments of Bode's original work together with all contemporary results.