Book excerpt: Infrared and Millimeter Waves is a series of books that compiles the work of several authors, with each volume focusing on certain aspects of infrared and millimeter waves, such as sources of radiation, instrumentation, and millimeter systems. This book is the first of a two-part volume that covers the coherent sources and applications of infrared and millimeter waves. The first chapter discusses sources and applications, while the second chapter provides a comprehensive discussion about spectroscopy of molecules by far-infrared laser emission. This is followed by a discussion of stark spectroscopy and frequency tuning in optically pumped far-infrared molecular lasers. Measured performance of gyrotron oscillators and amplifiers is also covered. This book will be of great use to researchers and professionals whose work involves infrared and millimeter waves.