  I discuss constraints on star--formation and AGN activity in massive galaxies
at z~1-3 using observations from the Spitzer Space Telescope. In particular I
focus on a sample of distant red galaxies (DRGs) with J-K>2.3 in GOODS-S. Based
on their ACS, ISAAC, and IRAC photometry, the DRGs have typical stellar masses
>10^11 Msol. Interestingly, the majority (>50%) of these objects have 24 micron
detections. If attributed to star formation, this implies SFRs of ~100-1000
Msol/yr. Thus, massive galaxies at z~1.5-3 have specific SFRs equal to or
exceeding the global average value at that epoch. In contrast, galaxies with
>10^11 Msol at z~0.3-0.75 have specific SFRs less than the global average, and
more than an order of magnitude lower than that at z~1.5-3. Thus, the bulk of
assembly of massive galaxies is largely complete by z~1.5. At the same time,
based on the X-ray luminosities and near-IR colors, as many as 25% of the
massive galaxies at z>1.5 host AGN, implying that the growth of supermassive
black holes coincides with massive-galaxy assembly. The analysis of
high-redshift galaxies depends on bolometric corrections between the observed
24 micron data and total IR luminosity. I review some of the sources of the
(significant) uncertainties on these corrections, and discuss improvements for
the future.
