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 1.7kV 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. The call for paper of the CPSS special issue on MMC was published in the late of 2019. We received many inquiries and a total of twelve submissions. Reviews were promptly organized by invited Associated Editors from China, Europe, and the United States. Reviewers are invited from around the globe. After two rounds of reviews, five papers were accepted. Because of the publication timing, one of the papers will appear in a later issue of TPEA. Each of these accepted papers addresses one particular challenge with innovative solutions. Few of these papers also showcase successful international collaborations on fundamental research of MMC. The paper by X. Xiong and her co-authors from the North China Electric Power University, the Aalborg University, and the State Grid Corporation of China, proposes a highly effective circulating current suppression method based on common-mode insertion indices with feed-forward sub-module capacitor voltage. The method suppresses all harmonic components in the circulating current and naturally maintains submodule capacitor voltage balancing between upper and lower arms. The paper by B. Li and his co-authors from the Harbin Institute of Technology presents a hybrid back-to-back connected MMC that features an ingenious constant dc link current control which enables variable dc input voltage for the motor side MMC, which helps to reduce voltage ripple on submodule capacitors at low motor speed. The paper by G. Liang and R. Zhu from the North China Electric Power University presents a method that can predict the radiated electromagnetic interference in the valve hall of the MMC station. The method divides the MMC into decoupled sub-circuits, which helps to improve the numerical stability and speed up the convergence process of simulations. The research is of importance to ensure the normal operation of MMC and the safety of engineers. The paper by F. Qin and his co-authors from the Shandong University and the Nanyang Technological University gives a comprehensive review of six-arm MMC and nine-arm MMC. The paper investigates the two converter topologies in terms of the number of submodules, current stress of semiconductor devices, maximum capacitor voltage ripple, and system power losses. The presented analysis and result can provide good reference values for designing high-performance MMC systems. We appreciate the effort from all authors who had submitted papers and we appreciate timely reviews from associated editors and reviewers, especially at this time of difficulty. Power electronics is like a LEGO game. Switches, capacitors and inductors are among major building blocks. MMC is one of crown jewels of this ultimate engineering LEGO game. We need master topology, control, EMI/EMC, thermal, etc., to make this game work. This special issue with its papers serves as an example of ongoing worldwide researches and implementations of MMC. We look forward to seeing more great papers on MMC in the CPSS Transactions on Power Electronics and Applications in the time yet to come.