Harmonic impedance characterizes the voltage response of a power system when it is subjected to the influence of high-frequency currents. The impedance is a key parameter of a power network and must be known to diagnose power system problems caused by high-frequency disturbances and to design disturbance-mitigation measures. Unfortunately, determining an operating power system’s harmonic impedances is very difficult; they must be measured when the system is energized. In fact, how to measure a power system’s high-frequency impedances has been a challenging and frequent research topic in the power engineering field. This thesis presents a measurement methodology that can determine the harmonic impedances and sources at both sides of utility-customer interface. This methodology is applicable to single-phase three-wire systems under energized conditions. A potential application of the method is to determine the harmonic contributions of the supply system and its customers at the interface points.