The International Celestial Reference System (ICRS) is the current standard celestial reference system adopted by the International Astronomical Union (IAU). Its origin is at the barycenter of the Solar System, with axes that are intended to "show no global rotation with respect to a set of distant extragalactic objects". This fixed reference system differs from previous reference systems, which had been based on Catalogues of Fundamental Stars that had published the positions of stars based on direct "observations of [their] equatorial coordinates, right ascension and declination" and had adopted as "privileged axes ... the mean equator and the dynamical equinox" at a particular date and time. The International Celestial Reference Frame (ICRF) is a realization of the International Celestial Reference System using reference celestial sources observed at radio wavelengths. In the context of the ICRS, a reference frame (RF) is the physical realization of a reference system, i.e., the reference frame is the set of numerical coordinates of the reference sources, derived using the procedures spelled out by the ICRS. More specifically, the ICRF is an inertial barycentric reference frame whose axes are defined by the measured positions of extragalactic sources (mainly quasars) observed using very long baseline interferometry while the Gaia-CRF is an inertial barycentric reference frame defined by optically measured positions of extragalactic sources by the Gaia satellite and whose axes are rotated to conform to the ICRF. Although general relativity implies that there are no true inertial frames around gravitating bodies, these reference frames are important because they do not exhibit any measurable angular rotation since the extragalactic sources used to define the ICRF and the Gaia-CRF are so far away. The ICRF and the Gaia-CRF are now the standard reference frames used to define the positions of astronomical objects. Reference systems and frames It is useful to distinguish reference systems and reference frames. A reference frame has been defined as "a catalogue of the adopted coordinates of a set of reference objects that serves to define, or realize, a particular coordinate frame". A reference system is a broader concept, encompassing "the totality of procedures, models and constants that are required for the use of one or more reference frames". Realizations The ICRF is based on hundreds of extra-galactic radio sources, mostly quasars, distributed around the entire sky. Because they are so distant, they are apparently stationary to our current technology, yet their positions can be measured