This book focuses on a fundamental feature of vacuum electronics: the strong interaction of the physics of electron beams and vacuum microwave electronics, including millimeter-wave electronics. The author guides readers from the roots of classical vacuum electronics to the most recent achievements in the field, exploring both the physics and the theory underlying electron beams and devices of vacuum high-frequency electronics. Special attention is devoted to the physics and theory of relativistic beams and microwave devices. Readers gain a deep understanding of the topic as well as the theory and applications of specific devices.