Theoretic fundamental principles, system design rules, high power microwave measurement and experimental data on virtual cathode oscillator, a type of high power microwave source, are presented in this book. Virtual cathode oscillator can work as a core part of electromagnetic missiles, electromagnetic bombs, high power pulse radars and high power jammers; furthermore, it can provide high power microwave source for accelerators and Tokamaks. The author conducted in-depth research in space charge limited current for coaxial vacuum diodes and developed an analytic expression for Langmuir-Blodgett law, which is a counterpart of Child-Langmuir law. Another highlight of this book is the microwave virtual cavity concept that the author proposed based on experimental data obtained while in Texas Tech University, which is an important contribution to traditional microwave resonance cavity theory. This book also explores high power microwave measurement and proposes a novel approach to monitor high power microwave pulses. In summary, engineers and scientists working on high power microwave sources and other relative areas will benefit from the book.