Addressing the current industry issues regarding unclear and logically imprecise definitions of grid-forming VSC and grid-following VSC, this chapter reclassifies VSC types and provides clear definitions based on whether VSC can independently establish grid-wide frequency. This chapter defines the interaction relationship between VSC and the connected power grid as two basic types: Grid Forming VSC and Grid Supporting VSC. This chapter covers: elements and classification of voltage source converter control; mathematical model of MMC in synchronously rotating coordinate system; dual-mode dual-loop controller design of MMC based on PLL; principles and applicable scenarios of zero-sequence third harmonic voltage injection for improving MMC performance; MMC controller design under unbalanced and distorted AC grid voltage conditions; PSL-based MMC controller design under balanced AC grid conditions; performance comparison between PLL-based and PSL-based controllers; analysis of phase-lock failure factors and performance improvement methods for phase-locked loop; MMC controller design when serving as grid-forming source for passive grid or renewable energy base grid; definition and significance of voltage toughness.