Many problems of geodynamics depend on spatial relationship of points and their temporal variations. To solve these problems it is convenient, but not necessary, to use a reference frame. To use a reference frame, a scheme is needed by which the coordinates of any point expressed in this frame could be obtained. Coordinates are hardly ever measured directly. Instead, they are computed from measurements of other quantities within the framework of a theory that relates the measured quantities with the coordinates. Such a scheme will be called a reference system. Reference systems have been realized using simple theories and reducing the measurements with the best available, but not always complete, theories. This geometric (static) method has been used to a great extend to define astronomic reference systems (star catalogues) and geodetic reference systems (geodetic datums), With space techniques, a method can be used based on dynamic principles. A space object moving according to a certain theory (assumed to be known) defines in a time dependent way the representative points. A reference system of this type is the WGS 72.