MODULAR multilevel converter(MMC), since its inception, is poised to change the landscape of medium and high voltage power electronics applications. Power electronics circuit topologies are centered around capacitors, inductors, or combinations of capacitors and inductors. Conventional multilevel converters are capacitor centered and have their advantages and limitations. For example, the neutral point clamped(NPC) multilevel converters and flying capacitor multilevel converters can be back-back connected but are not easily scalable to achieve higher voltage. The cascade multilevel converter has modular structure and good scalability but cannot be back-to-back connected. Compared to these three types of multilevel converters, MMC indeed is a remarkable hybrid circuit that has both capacitors centered sub-modules and arm inductors. It achieves good scalability and is feasible for back-back connections. In the last two decades, MMC has already been widely implemented in utility applications. Recently, medium voltage megawatt level MMC with 1.7 kV rated Silicon Carbide(SiC) devices has also been successfully demonstrated for motor drive applications. Nevertheless, MMC still presents many research challenges and opportunities in terms of circuit topology variations, sub-module capacitor reduction in all types of applications, integration of medium voltage wide bandgap power devices, electromagnetic interference and compatibility, implementation in solid state transformer to achieve high step down ratio and frequency multiplication, etc.