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The basic properties of atomic primary frequency standards are reviewed. A continuously running frequency source combined with counting, storage, and display devices results in a clock. Time scales are obtained by setting clocks with respect to a convenient origin. The accuracy of a primary frequency standard is a combined theoretical and experimental assessment of the uncertainties of all known possible sources of bias from the idealized definition. Recommended standard measures for the stability are the spectral density of fractional frequency fluctuations (frequency domain) and the two-sample no-dead time Allan variance (time domain). The operation of atomic frequency standards is based on various methods of particle interrogation to observe the transitions, particle confinement to obtain sufficient interaction time, and particle preparation to obtain the desired energy level populations. Passive resonator and active oscillator (maser) modes of operation are discussed; the former has some fundamental advantages. A review of the state of the art and current practice shows that cesium beam resonator standards have the best documented accuracy capability (5 × 10-13) and, in their commercial versions, are presently the best available clocks. There are three concepts related to time: time interval, date, and synchronization. In order to assign dates to events, time scales have to be established. The steadily growing need for a very precise and uniform time scale has resulted in a new internationally coordinated time scale, IAT. Frequency metrology will have increasing impact on both length and voltage metrology in the near future.