The use of space-geodetic techniques to study geodynamic processes began with Very Long Baseline Interferometry (VLBI) in the early 1970s. By measuring the delay in arrival time of the signal from distant celestial objects, the distances between stations that are hundreds of kilometres apart can be derived with millimetre accuracy. A review of the first 20 years of this technique is given by Ryan and Ma (1998). Around the world there are nowadays more than 100 VLBI stations. Another technique that has been available since the early 1970s is Satellite Laser Ranging (SLR). As the name implies, this technique determines the distance to a satellite by measuring the round trip time of a light pulse that is sent to the satellite (Degnan 1993). Today, there are about 60 SLR stations operational around the world.