After commenting on certain definitions related to both the terrestrial and celestial (quasi-inertial reference systems (CTS and CIS) to clarify terminology, the geodynamic requirements for such systems are reviewed. This is followed by a discussion of certain problematic aspects of the two systems. The article concludes with a list of required actions aimed to assure that the reference system issue is resolved early. The list is proposed for the joint IAG/IAU working group to be established in accordance with the Colloquium resolution printed elsewhere in this volume.