Flux modulation machines (FMMs) represent a groundbreaking class of electrical machines, distinct from conventional designs in their principles, structure, and characteristics. This chapter offers a systematic definition of FMMs, starting with an in-depth exploration of the term “modulation” and gradually differentiating FMMs from traditional machines. Building on this foundation, the chapter introduces various types of FMMs extensively studied in academia, providing concise descriptions of their performance characteristics and appropriate application areas.