  I discuss current observational constraints on the star-formation and
stellar-assembly histories of galaxies at high redshifts. The data on massive
galaxies at z<1 implies that their stellar populations formed at z>2, and that
their morphological configuration was in place soon thereafter. Spitzer Space
Telescope 24 micron observations indicate that a substantial fraction of
massive galaxies at z ~ 1.5-3 have high IR luminosities, suggesting they are
rapidly forming stars, accreting material onto supermassive black holes, or
both. I compare how observations of these IR-active phases in the histories of
massive galaxies constrain current galaxy-formation models.
