Within the past 5 years, relative-position measurements performed using space geodetic techniques, particularly satellite laser ranging (SLR) and very long baseline interferometry (VLBI), have provided a new tool for the study of geodynamics. With accuracies of 1 part in 108 and better, these measurements have been used to determine, among other things, present-day plate motions and deformation in western North America. However, the size and cost of SLR and VLBI systems have prohibited the establishment of extensive networks required to study many of the important problems in crustal dynamics. A technique much more suited for this purpose is radio-interferometric tracking of the satellites of the Global Positioning System (GPS). Though slowed initially by the high cost of instruments and limited satellite visibility in some parts of the world, the use of GPS measurements for geodynamics has rapidly expanded. In the 1990s we will likely see repeated measurements in most active regions of the world and the establishment of permanent, continuously monitored networks in several areas. There will also be increasing interest in the use of GPS as a component of satellite altimeter missions and as a monitor of global change in sea level.