  Reference systems and frames are crucial for high precision absolute
astrometric work, and their foundations must be well-defined. The current
frame, the International Celestial Reference Frame, will be discussed: its
history, the use of the group delay as the measured quantity, the positional
accuracy of 0.3 milliarcsec, and possible future improvements. On the other
hand, for the determination of the motion of celestial objects, accuracies
approaching 0.01 milliarcsec can be obtained by measuring the differential
position between the target object and nearby stationary sources. This
astrometric technique uses phase referencing, and the current techniques and
limitations are discussed, using the results from four experiments. Brief
comments are included on the interpretation of the Jupiter gravity deflection
experiment of September 2002.
