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After a brief survey of the state of circuit theory before World War I, the various directions of its development in the last 50 years are discussed, mainly in relation with applications to communication engineering. The early period of network design (1920-1925) was followed by the beginning of synthesis (1926-1935). The next steps were the development of feedback amplifier theory and insertion loss filter theory. The numerous new directions of research started during and after the second war are briefly mentioned. Finally recent progress is reported in formal realizability theory and in topological synthesis. A last section deals with nonlinear and linear variable circuits.