I start by general remarks on the background of the recommendations on space-time references which are submitted to you.

The need to consider time scales in a relativistic framework appeared more than 20 years age following the progress of atomic time standards. After long discussions, this led the IAU to define, In 1976, time scales which were designated, In 1979, as Terrestrial Dynamical Time (TDT) and Barycentric Dynamical Time (TDB). But soon afterwards difficulties in the interpretation of the definitions of TDT and TDB arose. It appeared that the source of these difficulties was the lack of a global approach to space-time reference systems. This point of view, first voiced by J. Lieske, gained acceptance. At the very beginning of the work of the WGRS Sub-Groups on Frames and Origins (SGFO) and on Time (SGT), It became clear the the primary mission of the SGFO and SGT was to jointly prepare general recommendations on space-time references on which they could base their specific recommendations.