  Photometric redshifts are studied with a template approach using data from
HDF-N and -S . The problem of aliasing in photometric redshift estimates is
investigated in some detail and found not to be a significant problem if at
least four photometric bands are available. The performance of the approach
presented here appears to exceed that of others in the literature.
  With reasonable restrictions, it is possible to determine the dust extinction
as well as the photometric redshift, provided five or more photometric bands
are available. An important result is that evolution of <A_V(z)> with redshift
is seen, with higher values than locally at z = 0.5-1.5, and lower values at z
> 2. This is consistent with current models for the star formation history of
the universe.
  Deconvolving the uv-to-ir seds into an old star and young star component
allows determination of M_* and \dot{M}_* for each galaxy, as well as z_{phot}
and A_V, provided that infrared photometric bands are available. The expected
trend of b = M_*/\dot{M}_* t_0 increasing to the past is seen. However there is
a great deal of scatter in the relation between b and sed type, showing that
the recent star-formation history is not very well correlated with the
long-term history of a galaxy.
  The 2800 A luminosity function and star-formation rate are calculated for a
large sample of HDF-N (2490) and HDF-S (28719) galaxies, using photometric
redshifts, for the redshift range 0.2-5. The star-formation rates agree
reasonably well with those from a variety of other uv, $H_{\alpha}$, infrared
and submillimetre estimates, and with star-formation histories used to model
optical, infrared and submillimetre source-counts.
