The large number of control devices which had been invented over the previous two centuries led, by about 1930, to the conception of a control theory with fundamental principles basic to all forms of control. The last war saw the use of this theory to develop sophisticated mechanisms of high precision. The study gives the development of the science and engineering of control over the last 40 years. In engineering, the increasing importance of the effects of non-linearity is emphasised, and the spinoff resulting in unconventional electrical machines is described including the development of magnetic levitation. During this period control theory has also been introduced into a wide variety of other branches of knowledge. The greatest impact has probably been in medicine, with the body now being regarded functionally as a complex, dynamic control system. The heart–lung combination was eventually seen to be a ‘self-adaptive’ system, and the study had an effect on engineering design. Other areas described include sociology, economics and management. The basic tenets of the accuracy of measurement, the speed of measurement and ‘dead time’, well established in engineering, are shown to be equally fundamental in management and economics.