There have been several recent widespread oscillation events in the eastern and   western interconnections, and in Europe, which were caused by forced oscillations resonating with system modes. For instance, large MW oscillations were seen throughout the eastern interconnection on January 11, 2019, from a 0.25 Hz forced oscillation at a power plant in Florida. These events are related to the phenomenon of interarea resonance when a forced oscillation interacts with system interarea modes with close-by frequencies. The resonance effect can lead to long distance propagation of a forced oscillation, even when the associated system mode is well-damped, depending on the sensitivity of the source location of the forced oscillation. This talk will summarize the mathematical theory of interarea resonance based on linear system analysis and will present several signal processing algorithms for recognizing and analyzing such events online using synchrophasor measurements. The methodology will be illustrated on recent system oscillation events in the North American interconnections.