Until recently, VLBI targets have been drawn almost exclusively from the brightest and most compact radio sources in the sky, with typical flux densities well in excess of a few tens of mJy. These sources are predominantly identified with Active Galactic Nuclei (AGN), located at cosmological distances (z ~ 1). Exotic but also rather rare, these luminous AGN systems have been studied in great detail by VLBI over the last 3 decades, producing many front-line discoveries along the way (see this volume and references therein).
However, in recent years, connected arrays (such as the VLA, WSRT, ATCA and MERLIN) have also began to focus a significant fraction of their time towards understanding the nature of the faint radio sky - sometimes observing the same field for many days or even weeks at a time. At these microJy noise levels, the radio sky literally "lights up", and a new population of vigorous star forming galaxies begin to dominate the radio source counts (Fomalont et al. 1997, Richards et al. 1998, Muxlow et al. 1999, Richards 2000, Garrett et al. 2000a, Norris et al. 2000). For many astronomers (usually radio astronomers!) it comes as some surprise that a well calibrated VLBI array, composed of the largest telescopes in the world, can also contribute to our understanding of this sub-mJy and microJy radio source population. Nevertheless, these are the facts, as recently demonstrated by the simultaneous detection of 3 sub-mJy radio sources in the Hubble Deep Field North (HDF-N) by the European VLBI Network (Garrett et al. 2001).
In this lecture I will attempt to summarise what is currently known about the general properties of the faint sub-mJy and microJy radio source population, as determined from deep multi-wavelength studies of the HDF-N. In particular, I will try to provide a VLBI perspective, describing the first deep, wide-field, VLBI pilot observations of the HDF, together with a summary of the main results. The role VLBI can play in future high resolution studies of faint radio sources will also be addressed.