This book is written for the Undergraduate, Postgraduate and Doctoral students specializing in Microwave and Millimeter wave Avalanche Transit Time Devices and Oscillators. This book provides a clear insight of the device Physics and its application in Oscillator circuits. The main features of this book are Modelling and simulation of IMPATT Devices, their fabrication and characterization and high frequency limitation. A separate chapter is devoted on IMPATT Oscillator and amplifier. Finally the emerging application of the device as a potential Terahertz source is included to generate interest to the researchers working in this field.