Abstract
The IAU has introduced the International Celestial Reference System (ICRS), the Celestial Intermediate Pole (CIP), and the Celestial Ephemeris Origin (CEO) in order to be consistent with the improved accuracies currently achieved and expected in the future. While the bases of the International Celestial Reference System (ICRS) are different from previous reference systems, the astrometric definitions and methods remain similar and have a number of simplifications. The changes are from a dynamical to a kinematic defining motion, from bright stars to extragalactic reference sources, from a precessing equinox to a more stable fiducial point, and from a reference frame moving with time to a fixed reference frame independent of date. The new definitions necessary for the application of the ICRS to astronomy and astrometry are given and the methods for reducing observations are discussed. Recommendations are given concerning the forms of data to be provided and the reference frames to be used in various cases.
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