Abstract: Benefited from flux modulation effects, flux modulation machine (FMM) has the advantages of high torque density, which has become the promising candidate for the low-speed high-torque direct-driven applications, such as, wind generation, marine propulsion, and industry driving, and etc. In this paper, the origin of FMMs is introduced. Then, the development and improvement process of the basic theory and analysis method of FMMs are overviewed. Next, the classification and working principle of FMMs, as well as the improvement method of key electromagnetic performance, including torque density and power factor, are summarized. Finally, the main research results in the field of FMMs are summarized, and the technical challenges and future development directions of this type of machines are discussed.