  Multi-epoch radio-interferometric observations of young stellar objects can
be used to measure their displacement over the celestial sphere with a level of
precision that currently cannot be attained at any other wavelength. In
particular, the accuracy achieved using carefully calibrated, phase-referenced
observations with the 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 percents. Taking
advantage of this situation, we have initiated a large project aimed mainly at
measuring the distance to the nearest regions of star-formation (Taurus,
Ophiuchus, Perseus, etc.). Here, we will present the results for several stars
in Taurus and Ophiuchus, and show that the accuracy obtained is already more
than one order of magnitude better than that of previous estimates. The proper
motion obtained from the data can also provide important information,
particularly in multiple stellar systems. To illustrate this point, we will
present the case of the famous system T Tauri, where the VLBA data provide
crucial information for the characterization of the orbital path.
