In this book, an integrated systematic method (ISM), which combines the nonlinear hybrid method (NHM) based on the single nonlinear measured signal and the complex orthogonal decomposition (COD) based on the ensemble measurement matrix, is proposed to identify the dominant oscillation mode based on wide area measurement system (WAMS). The NHM is used to analyze the single measured signal from the nonlinear view and the COD is performed on the complex ensemble measurement matrix to find the optimal complex orthogonal modes (COMs) according to the energy distribution in wide area. The NHM and COD work in a way of parallel and complementary manner. The near real-time application of ISM combing with the sliding time window is researched based on the instantaneous computation capability of NHM and recursive algorithm of the COD. The research activities of this book include signal processing methodology, power system oscillation monitoring and dynamic stability analysis.