This book exhaustively explains the fundamental physical and theoretical principles underlying microwave and millimeter wave active devices. Both vacuum and solid state devices are suitably discussed. The book begins by highlighting the applications of microwaves and various types of devices. It then explains vacuum devices including gyrodevices and other high power sources. Various two and three terminal solid state devices are then discussed.These include HBTs, HFETs and RTDs. The text is amply illustrated through a large number of suitable diagrams and worked out examples. Practice problems, review questions and extensive references are also given at the end of each chapter. The book would serve as an exhaustive text for both undergraduate and postgraduate students of physics and electronics. About The Prof. M.L. Sisodia, Advisor and Prof., ECE, Global Institute of Technology, Sitapur, Jaipur, obtained his Master`s degree from University of Rajasthan, Jaipur, M.Tech. from IIT, Mumbai and Ph.D. (Engg.) from University of Rajasthan while working at CEERI, Pilani. Prof. Sisodia retired as Founder Director, UGC Academic Staff College, University of Rajasthan. He also worked as visiting Scientist/Professor in the Department of Electronics & Communication Engineering, Institute of Technology, BHU, Varanasi where he assisted in establishing High Power Microwave Tube Research Centre and design and development of 200 watt TWT under DoE project. He has published more than 100 research papers in the area of microwave vacuum and solid state devices, microwave components, microwave spectroscopy and education. So far 8 students have obtained their Ph.D. degree under his guidance. He is the author of Microwave Circuits and Pass