This book is the first English book to present the theory of flux modulation and name the flux modulation machine family. It explains the phenomenon of flux modulation in electrical machines based on the well-known "modulation" in power electronics; clarifies the difference and connection between different flux modulation machines through the topological evolution, comparatively analyzes their commonality and individuality characteristics under the same theoretical framework; and finally proposes the R&D ideas and development frontiers of high-performance machine topologies. The flux modulation theory is the main feature of this book, which is the expansion of the classical machine theory, providing readers in the field of electrical machines with a new vision and more design options.