In power system studies, some parameters cannot be measured directly by using the currently existing power meters. These power system parameters include the power system Thevenin impedance, harmonic impedances, zero sequence impedance, and the load model parameters. The power system’s Thevenin and harmonic impedance at a load bus is an important parameter for power system planning and operation as well as for harmonic mitigation, prediction of system resonance, and harmonic propagation studies. Furthermore, the load parameters in power systems play a significant role in power system planning, control, and stability analysis. This book presents newly developed and verified algorithms for online measurement and monitoring of these power system parameters. The algorithm proposed for monitoring the system’s Thevenin, harmonic, and zero sequence impedance parameters, uses the natural variations of the loads connected to the substations. The proposed algorithm for monitoring of load model parameters uses the voltage and current waveforms captured during the operation of the under load tap change transformers installed in the distribution substations.