This book describes the fundamental principles of designing integrated circuits used for sensors that operate by scavenging energy from the environment. Although energy scavenging and self-powering is an area which spans several engineering and scientific disciplines, this book focuses on fundamental principles of energy scavenging circuits and therefore should be accessible to any researcher working in the area of integrated circuits. This book will be a valuable reference for anyone interested in designing sensors and systems that can operate without batteries by scavenging ambient energy. It will also be beneficial to designers working on ultra-low power analog and digital signal processing circuits that can operate at fundamental limits of energy-efficiency and energy-scavenging.