  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. 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 is also addressed.
