The International Celestial Reference Frame (ICRF), is intended for adoption be the International Astronomical Union (IAU) as the conventional celestial system, under the name International Celestial Reference System (ICRS). (See http://maia.usno.navy.mil/iauc19/iaures.html#B5). A Working Group on Reference Frames (WGRF), with the participation of International Earth Rotation Service (IERS), was appointed by the IAU to accomplish the task of constructing the ICRF. The WGRF set up a list of 608 extragalactic radio sources uniformly distributed on the sky and evaluated their J2000.0 Very Long Baseline Interferometry (VLBI) coordinates in the ICRS by applying a no rotation constraint with respect to an earlier realization of the IERS Celestial reference system, RSC(IERS) 95 C 02 . The first realization of the ICRF has J2000.0 coordinates of 608 objects and is named RSC(WGRF)95 R 01. Radio sources in RSC(WGRF)95 R 01 are divided into three categories (see file "guide.txt"): defining, candidate and other sources. In order to provide radio source coordinates to link to the Hipparcos stellar reference frame to the ICRS, the WGRF issued a shorter version of the ICRF, RSC(WGRF) 95 R 02. It is a subset of RSC(WGRF) 95 R 01 that contains J2000.0 coordinates of 253 radio sources covering the declination range -81 degrees to +85 degrees. The 212 defining sources of RSC(WGRF)95 R 01 included 17 Hipparcos link objects. An additional 41 other radio sources had to be added RSC(WGRF) 95 R 02 for the link. The median positional accuracy of RSC(WGRF) 95 R 02 is 0.4 mas. The other 357 sources of RSC(WGRF) 95 R 01 have coordinates with lower precision (median= 0.8 mas) but they are still consistent with the ICRS. The direction of the axes of the ICRF are consistent with those of the FK5 system within the uncertainties of the latter. On the basis of Fricke's (1982A&A...107L..13F) and Schwan (1988) considerations, the uncertainty of the pole position relative to the mean pole at J2000.0 is +/-50 mas. Studies by Morrison et al. (1990A&A...240..173M) and Lindegren et al. (1995A&A...304...44L) have shown that the FK5 origin of right ascensions has an uncertainty of the order of +/-100 mas. The accuracy of the tie of the Hipparcos stellar frame to ICRS is +/-0.6 mas at the Hipparcos mean epoch of observation (1991.25) and +/-0.25 mas/year for the time evolution. The tie between the dynamical planetary frame and ICRS is known within +/-3 mas (Folkner et al. 1994A&A...287..279F). Abstract of the 1998AJ....116..516M paper: A quasi-inertial reference frame is defined based on the radio positions of 212 extragalactic sources distributed over the entire sky. The positional accuracy of these sources is better than about 1mas in both coordinates. The radio positions are based upon a general solution for all applicable dual-frequency 2.3 and 8.4GHz Mark III very long baseline interferometry data available through the middle of 1995, consisting of 1.6 million pairs of group delay and phase delay rate observations. Positions and details are also given for an additional 396 objects that either need further observation or are currently unsuitable for the definition of a high-accuracy reference frame. The final orientation of the frame axes has been obtained by a rotation of the positions into the system of the International Celestial Reference System and is consistent with the FK5 J2000.0 optical system, within the limits of the link accuracy. The resulting International Celestial Reference Frame has been adopted by the International Astronomical Union as the fundamental celestial reference frame, replacing the FK5 optical frame as of 1998 January 1.