Reference systems and frames are crucial for high precision absolute astrometric work, and their foundations must bewell-defined. The current frame, the International CelestialReference Frame, will be discussed: its history, the use of the groupdelay as the measured quantity, the positional accuracy of 0.3 mas,and possible future improvements. On the other hand, for thedetermination of the motion of celestial objects, accuraciesapproaching 0.01 mas can be obtained by measuring the differentialposition between the target object and nearby stationary sources.This relative astrometric technique uses phase referencing, andthe current techniques and limitations are discussed, usingthe resultsfrom four experiments. Brief comments are included on theinterpretation of the Jupiter gravity deflection experiment ofSeptember 2002.