  Multi-epoch radio-interferometric observations of young stellar objects can
be used to measure their displacement over the celestial sphere with a level of
accuracy that currently cannot be attained at any other wavelength. In
particular, the accuracy achieved using carefully calibrated, phase-referenced
observations with Very Long Baseline Interferometers such as NRAO's Very Long
Baseline Array is better than 50 micro-arcseconds. This is sufficient to
measure the trigonometric parallax and the proper motion of any radio-emitting
young star within several hundred parsecs of the Sun with an accuracy better
than a few percent. Using that technique, the mean distances to Taurus,
Ophiuchus, Perseus and Orion have already been measured to unprecedented
accuracy.
  With improved telescopes and equipment, the distance to all star-forming
regions within 1 kpc of the Sun and beyond, as well as their internal structure
and dynamics could be determined. This would significantly improve our ability
to compare the observational properties of young stellar objects with
theoretical predictions, and would have a major impact on our understanding of
low-mass star-formation.
