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This chapter describes the oscillatory dynamics in the wide-area power system by using acquired monitoring data with phasor measurement units (PMUs). Particularly, interarea low-frequency oscillations in Japan and Southeast Asia power systems are investigated by adopting the band-pass filtering based on the fast Fourier transform technique. Since both systems have the longitudinal configuration, the low-frequency mode oscillates in the opposite phase between both ends of the power network. The oscillatory dynamics can be captured successfully by the wide-area phasor measurements. The chapter presents a method to identify the dominant mode by using measured phasor fluctuations via the wide-area monitoring system (WAMS) in the normal operating condition. Dynamic characteristics of power system oscillations can be investigated based on the measured PMU data. The characteristics of such dominant modes should be analyzed to maintain power system stability and reliability.