  I review a technique for interpreting faint galaxy data which traces the
evolution with cosmic time of the galaxy luminosity density, as determined from
several deep spectroscopic samples and the HDF. The method relies on the rest
frame UV and near-IR continua of galaxies as indicators, for a given IMF and
dust content, of their instantaneous SFR and total stellar mass, and offers the
prospect of addressing in a coherent framework an important set of subjects:
cosmic star formation history, dust in primeval galaxies, shape of the IMF,
stellar mass-to-light ratios of present-day galaxies, extragalactic background
light, Type II supernovae and heavy element enrichment history of the universe.
The global spectrophotometric properties of field galaxies are well fit by a
simple stellar evolution model, defined by a time-dependent SFR per unit
comoving volume, a universal IMF which is relatively rich in massive stars, and
a modest amount of dust reddening. The model is able to account for the entire
background light recorded in the galaxy counts down to the very faint magnitude
levels probed by the HDF, and produces visible mass-to-light ratios at the
present epoch which are consistent with the values observed in nearby galaxies
of various morphological types. The bulk (>60%) of the stars present today
formed relatively recently (z<1.5), consistently with the expectations from a
broad class of hierarchical clustering cosmologies, and in good agreement with
the low level of metal enrichment observed at high redshifts in the damped
Lyman-alpha systems. Throughout this review I emphasize how the poorly
constrained amount of starlight that was absorbed by dust and reradiated in the
far-IR at early epochs represents one of the biggest uncertainties in our
understanding of the evolution of luminous matter in the universe.
