  Astrometry is a powerful technique in astrophysics to measure
three-dimensional positions of stars and other astrophysical objects, including
exoplanets and the gravitational influence they have on each other.
Interferometric astrometry is presented here as just one in a suite of powerful
astrometric techniques, which include space-based, seeing-limited and
wide-angle adaptive optics techniques. Fundamental limits are discussed,
demonstrating that even ground-based techniques have the capability for
astrometry at the single micro-arcsecond level, should sufficiently
sophisticated instrumentation be constructed for both the current generation of
single telescopes and long-baseline optical interferometers.
