Primary frequency standards at NIST

The development of primary frequency standards at the National Institute of Standards Technology (NIST) is traced, and generations of atomic frequency standards described: cesium-beam standards, cesium-fountain standards, and stored-ion standards. The uncertainty of the present NIST frequency standard is 1.7 parts in 10/sup 15/, but the prospects for substantial improvement are high. Finally, the uncertainties of the most recent NIST standards are displayed relative to the uncertainties of standards of several other countries.