The idea of using extragalactic objects to define the celestial reference frame is quite old, having been discussed by Herschel and Laplace some two hundred years ago, even before there was proof that such objects existed. Any object sufficiently distant would have no detectable proper motion, thus avoiding one of the chief complications of the stellar reference frame discussed in Chapter 1. The realization of such a reference frame, however, has been dependent on scientific discovery, technological advances, and commitment of resources to a now perceived need.