Principles of Microwave Circuits is an unabridged reprint of the book first published in 1948 by McGraw Hill as Volume 8 of the MIT Radiation Laboratory Series. Since the original publication of this book a number of errors have been brought to our attention. Corrections of these errors are incorporated in this edition. In the engineering application of low-frequency currents an important step forward was the development of the impedance concept and its utilization through the theory of linear networks. It was almost inevitable that this concept would be generalized and become useful in the application of microwaves. This volume is devoted to an exposition of the impedance concept and to the equivalent circuits of microwave devices. It is the intention to emphasise the underlying principles of these equivalent circuits and the results that may be obtained by their use. Specific devices are not discussed except as illustrations of the general methods under consideration. The results of such calculations that had been performed up to that time are compiled in Vol. 21 of the Electromagnetic Waves Series the Waveguide Handbook edited by N. Marcuvitz and these results are used freely. Although the work of the Radiation Laboratory at MIT was the development of military radar equipment the principles discussed in this volume can be applied to microwave equipment of all kinds.