This chapter is about measuring voltage and current phasors and synchrophasors. Phasor calculation, time synchronization, data concentration, and different types of phasor measurement units are discussed. Measuring phase angles, impact of the changes in the system frequency on measuring phase angles, and the applications of phase angle measurements are covered. Measuring the rate of change of frequency and the applications of synchronized frequency measurements are discussed. Relative phase angle difference and phasor differential and their applications, such as in the analysis of wide-area oscillations; and in identifying the location of events are explored. Analysis of events in phasor measurements; including feature selection and classification are covered. Three-phase phasor measurements, unbalanced phasor events, phase identification, and the application of symmetrical components are discussed. State estimation, parameter estimation, and topology identification in power systems using phasor measurements are presented. Other subjects that are covered in this chapter include accuracy in phasor measurements, performance classes, steady-state performance, and dynamic performance.