Power systems are the high-order time-varying nonlinear systems. In power systems, especially in interconnected ones, low-frequency oscillations (LFO) occur from time to time leading to accidents with severe consequences. Modern LFO monitoring systems based on Phasor Measurement Units (PMU) are designed to timely identify the danger of the power system stability violation. This monograph chapter presents some results of studies various signal analysis methods to their use in real-time to identify the modes of electromechanical oscillations in power systems. Comparison application results of different signal analysis methods oriented to real-time use is presented. It is shown that when these methods are using in real-time under certain conditions the differing identification results of LFO modes may be obtained. To provide required reliability of the LFO modes identification few the most suitable for real-time application in LFO monitoring systems methods were selected. Taking into account the possibility of obtaining by such methods differing identification results the special procedure was proposed to process and to generalize corresponding identification results. Such approach makes it possible to obtain adequate estimates of LFO modes parameters more reliably. Two examples of the LFO analysis using PMU data are given. During such analyzing the ensemble of selected methods and the data obtained from PMU were used. These LFO arose in the Interconnected Power System of Ukraine on February 16, 2016 and on February 18, 2017.