Flexible alternating current transmission system (FACTS) and multi-line FACTS controllers play an important role in electrical power transmission systems by improving power quality and increasing power transmission capacity. These controllers are nonlinear and highly complex. Therefore, during transient conditions, it is very difficult to use conventional time and frequency domain techniques alone to determine the precise dynamic behavior of the harmonics introduced into the system by these controllers. In particular, the time-varying nature of the harmonic components is not captured by these techniques. This book consists of new dynamic models for seven important and popular FACTS controllers using a technique called the dynamic harmonic domain method. These models are more accurate than existing models. The models were simulated in the presence of disturbances to show the evolution in time of the harmonic coefficients and power quality indices. This detailed information about dynamic harmonic response is useful for power quality assessment and can be used as a tool for analyzing the system under the steady state and transient conditions and designing better control circuits.