This chapter examines low-frequency power transmission systems using the Modular Multilevel Matrix Converter (M3C). It covers the principles and applications of low-frequency power transmission, the M3C's standard structure and nomenclature, and its basic mathematical model. The discussion includes the M3C's equivalent circuit, the relationship between arm currents and input–output currents, and the set-average values of submodule capacitor current and voltage. The chapter also details methods for determining the number of cascaded submodules in M3C arms, submodule capacitance values, and arm reactor parameters. Furthermore, it explores how low-frequency-side frequency selection affects submodule capacitance values. The chapter concludes with comprehensive coverage of M3C control systems, including the overall controller design, input and output controller designs, circulating current suppression, and arm control strategies using nearest level modulation for submodule voltage balancing.