I review recent observational and theoretical progress in our understanding of the cosmic evolution of luminous sources .
Through a combination of deep HST imaging , Keck spectroscopy , and COBE background measurements , important constraints have emerged on the emission history of the galaxy population as a whole .
A simple stellar evolution model , defined by a star-formation density that rises from z = 0 to z \approx 1.5 , a universal Salpeter IMF , and a moderate amount of dust with A _ { V } = 0.23 mag ( A _ { 1500 } = 1.2 mag ) , is able to account for most of the optical-FIR extragalactic background light , and reproduces the global ultraviolet , optical , and near-IR photometric properties of the universe .
By contrast , a star-formation density that stayed roughly constant at all epochs appears to overproduce the local K -band luminosity density .
While the bulk of the stars present today formed relatively recently , the existence of a decline in the star-formation density above z \approx 2 remains uncertain .
If stellar sources are responsible for photoionizing the intergalactic medium at z \approx 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 \approx 3 .
A population of dusty AGNs at z \mathrel { \hbox to 0.0 pt { \lower 3.0 pt \hbox { $ \mathchar 536 $ } \hss } \raise 2.0 pt% \hbox { $ \mathchar 316 $ } } 2 could make a significant contribution to the FIR background if the accretion efficiency is \sim 10 \% .