Input-output feedback stability and robustness, 1959-85

The literature on input-output feedback is too large to do justice into a single article. Here we concentrate on the formative years ending in 1985 and leave the subsequent story to be told elsewhere. It is not a comprehensive survey of the literature or even of the most important papers. Rather, it is an attempt to describe events that marked the turning points. The period under scrutiny can be divided into roughly two parts; interest in nonlinear stability dominated the first part and robustness the second.