  I review recent observational and theoretical progress in our understanding
of the cosmic evolution of luminous sources. Largely due to a combination of
deep HST imaging, Keck spectroscopy, and COBE far-IR background measurements,
new constraints have emerged on the emission history of the galaxy population
as a whole. Barring large systematic effects, the global ultraviolet, optical,
near- and far-IR photometric properties of galaxies as a function of cosmic
time cannot be reproduced by a simple stellar evolution model defined by a
constant (comoving) star-formation density and a universal (Salpeter) IMF, and
require instead a substantial increase in the stellar birthrate with lookback
time. 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 history of the transition from the cosmic
`dark age' to a ionized universe populated with luminous sources can constrain
the star formation activity at high redshifts. 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 dusty, Type II
AGNs at z<2 could make a significant contribution to the FIR background if the
accretion efficiency is of order 10%.
