In the twenty-five years between 1930 and 1955 crucial changes in our understanding of feedback control systems occurred. The history of these developments is traced in this book. Feedback control devices were used for general industrial control, in process control, in aircraft and ships, in the telephone system and in analogue computing systems. The significant developments that occurred during the 1930s in several of these areas are analysed in detail. During the Second World War the ideas and techniques that had been developed in disparate areas were brought together to form what we now know as the classical frequency response methods of analysis and design. Work on methods for dealing with non-linear systems, sampled-data systems and stochastic systems began. The immediate post-war years saw the consolidation and dissemination of the classical methods and the addition of the root locus method for analysis and design. The final chapters cover the beginnings of so-called modern control with the introduction of state-space methods of analysis and design. As well as being of interest to engineers the book is also relevant to historians concerned with social, economic and labour history.