  There has been remarkable progress recently in both observational and
theoretical studies of galaxy formation and evolution. Largely due to a
combination of deep Hubble Space Telescope imaging, Keck spectroscopy, and COBE
far-IR background measurements, new constraints have emerged on the emission
history of the galaxy population as a whole. While the bulk of the stars
present today appears to have formed relatively recently, the existence of a
decline in the star-formation density above z=2 remains uncertain. The study of
the transition from the cosmic `dark age' to an ionized universe populated with
luminous sources can shed new light on the star formation activity at high
redshifts, and promises answers to some fundamental questions on the formation
of cosmic structures. If stellar sources are responsible for photoionizing the
intergalactic medium at z=5, the rate of star formation at this epoch must be
comparable or greater than the one inferred from optical observations of
galaxies at z=3. A population of quasars at z<2 could make a significant
contribution to the extragalactic background light if dust-obscured accretion
onto supermassive black holes is an efficient process.
