1. This book gives the reader a comprehensive and in-depth introduction to the variable gain control and details a series of related innovative technologies. Instead of reading various relevant books, this book enables readers to establish an overall understanding of variable gain control methods for different systems. 2. The goal of this book is to interpret complex control knowledge in simple terms. It is easy to understand and needs little background knowledge. 3. It highlights the practical applications of variable gain control in energy conversation. The designed variable gain control has a simple form and is convenient for engineering applications. 4. Discussion remarks are provided in each chapter with new contributions to emphasize the novelty of the presented design and analysis methods. Moreover, a number of interesting remaining problems in the related topics, which are worthy of further investigation, are discussed in this book. 5. Simulation results are given in each chapter, sometimes in a comparative manner, to show the effectiveness of these methods.