Book excerpt: Wide area monitoring, protection and control systems (WAMPACs) have been recognized as the most promising enabling technologies to meet challenges of modern electric power transmission systems, where reliability, economics, environmental and other social objectives must be balanced to optimize the grid assets and satisfy growing electrical demand. To this aim WAMPAC requires precise phasor and frequency information, which are acquired by deploying multiple time synchronized sensors, known as Phasor Measurement Units (PMUs), providing precise synchronized information about voltage and current phasors, frequency and rate-of-change-of-frequency.