This chapter discusses fast electromagnetic transient simulation methods for MMCs. The main approach used is the diakoptics-based interface variable equation approach for MMCs. From this foundation, two fast electromagnetic transient simulation methods are presented: the submodule Thevenin equivalent fast simulation method and the arm Thevenin equivalent fast simulation method. The chapter outlines key aspects of the diakoptics-based interface variable equation approach, including the problem origins, electromagnetic transient simulation implementation, and discretized companion models of energy storage elements. The chapter concludes with a comparative analysis of common simulation methods and their applicability.