Originally published in 1947, this book was written to provide an introductory survey of the developments in electromagnetic waves. Although the propagation of electromagnetic waves in metal tubes - wave-guides - had been studied for over fifty years prior to the publication of this book, the subject was primarily of theoretical interest. The treatment in this book reflects the movement away from the theoretical to a more practical interest in waves during the war, with the development of the first micro-wave radar equipment in 1940–1. The first six chapters are based on courses on microwave techniques, which were given during the war at the Radar School of the Telecommunications Research Establishment, whilst chapter seven is a mathematical treatment of the subject. This book will be of great value to scholars of the history of physics and electromagnetics.