Book excerpt: The variable gain control method is a new construction technique for the control of nonlinear systems. By properly conducting state transformation that depends on the variable gains, the control design problem of nonlinear systems can be transformed into a gain construction problem, thus effectively avoiding the tedious iterative design procedure. Different from the classical backstepping method and forwarding design method, the structure of variable gain control is simpler in the sense that fewer design parameters are required, facilitating the improvement of system control performance. To highlight the learning, research, and promotion of variable gain control, Variable Gain Control and Its Applications in Energy Conversion is written based on the research results of peers at home and abroad and combining our latest research. This book presents innovative technologies for designing variable gain controllers for nonlinear systems. It systematically describes the origin and principles of variable gain control for nonlinear systems, focuses on the controller design and stability analysis, and reflects the latest research. In addition, variable gain control methods applied to energy conversion are also included. Discussion remarks are provided in each chapter highlighting new approaches and contributions to emphasize the novelty of the presented design and analysis methods. In addition, simulation results are given in each chapter to show the effectiveness of these methods. It can be used as a reference book or a textbook for students with some background in feedback control systems. Researchers, graduate students, and engineers in the fields of control, information, renewable energy generation, electrical engineering, mechanical engineering, applied mathematics, and others will benefit from this book.