This tutorial examines advanced FACTS controllers based on the use of turn-on turn-off power semiconductors such as GTOs and MCTs. GTOs are the only commercially available power semiconductor suitable for large advanced FACTS controllers applicable to electric power transmission. However; the best available GTOs suitable for high-voltage, high-power applications have switching losses approximately 10-20% greater than an equivalently rated conventional thyristor Switching frequencies much greater than the line frequency are impractical with GTOs due to high losses and electrical characteristics. However other more advanced power semiconductors under development will hopefully not have these drawbacks.