This chapter provides a definition of Flexible Transmission Technology. In the chapter, the advantages of VSC-HVDC Transmission Technology compared to traditional HVDC transmission technology are described; the benefits of Modular Multilevel Converter (MMC) over two-level or three-level voltage source converters are discussed; and the basic control strategies of VSC-HVDC transmission systems are explained. This chapter also discusses the application scenarios of VSC-HVDC transmission technology, including point-to-point transmission, back-to-back asynchronous interconnection, back-to-back semi-asynchronous and semi-synchronous grid partitioning, and the construction of DC grids. The Flexible AC Transmission Technology (FACTS) based on cascaded-submodule converters is explored. The characteristics and application scenarios of STATCOM, UPFC, and M3C are outlined.