Today we have a number of technique-specific global and regional space geodetic networks. We mention in particular the IGS network (International GPS Service), the ILRS network (International Laser Ranging Service) and the IVS network (International VLBI Service). There also is a limited number of regional space geodetic networks, set up either for reference frame purposes or for monitoring crustal deformation. Examples are the EUREF network, the multi-technique networks of APSG and of the Japanese Key Stone Project (KSP).

Collocating networks always was an important issue in Space Geodesy, in particular within CSTG, the Commission VIII of IAG and B.2 of COSPAR on the Coordination of Space Techniques for Geodesy and Geodynamics. We give an overview of the work that was and is being performed in this area within CSTG in the years 1995-1999. We report in particular on the creation of the ILRS and the IVS, give a brief overview of the status of the IGS, and we give a status report on the creation of the ISGN, the International Space Geodetic Network, which is about to emerge as a coordinating tool for these individual space geodetic networks.

Unifying space geodetic networks does not make sense if this is not accompanied by the development of mathematically correct and operationally efficient analysis combination techniques in Space Geodesy. This is a delicate and technically demanding area but there is striking evidence that we will get a full picture of the Earth system only through a combination of all space geodetic (and other) techniques in the analysis.

It is in exactly this area that projects like the Key Stone Project are capable of playing an outstanding role in Space Geodesy. This network may be considered as a laboratory providing answers to many, if not most technical problems in the context of combination.