VLBI Astrometry

VLBI astrometry allows coordinates of celestial radio sources to be determined with submilliarcsecond accuracy. In particular, with differential VLBI astrometry, the standard errors of relative positions and proper motions can be as small as 10 microarcseconds and 10 microarcseconds per year, respectively. I will review astrometric observations of AGN, M81*, a supernova, pulsars, and radio stars, including the guide star, HR 8703, for the NASA/Stanford gyroscope relativity mission, Gravity Probe B.