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Power system stability and security assessment has been considered as an important control issue for secure system operation over many years. In this chapter, for the purpose of power system stability/security analysis and control synthesis, some graphical tools are introduced, that is, angle-voltage deviation, voltage-frequency deviation, frequency-angle deviation, and electromechanical wave propagation graphs. It explains the necessity of using the descriptive and graphical tools rather than pure analytical and mathematical approaches in wide-area power system stability and security issues. The improved situational awareness provided by phasor measurement units (PMUs) and wide-area monitoring systems (WAMSs) opens further improvements of graphical analysis, identification, and control tuning methods, providing robustness against unforeseen disturbances. The main purpose of these tools is easy monitoring to prevent instability and maintain an interconnected operation of the power system.