  I discuss constraints on star formation and AGN in massive, red galaxies at
z~1-3 using Spitzer observations at 3-24 micron. In particular I focus on a
sample of distant red galaxies (DRGs) with J - K > 2.3 in the southern Great
Observatories Origins Deep Survey (GOODS-S) field. The DRGs have typical
stellar masses >10^11 solar masses. Interestingly, the majority (>50%) of these
objects have 24 micron flux densities >50 micro-Jy. At these redshifts massive
galaxies undergo intense (and possibly frequent) IR-active phases, which is in
constrast to lower-redshift massive galaxies. If the 24 micron emission in
these z~1-3 galaxies is attributed to star formation, then it implies star
formation rates (SFRs) in excess of ~100 solar masses per year. These galaxies
have specific SFRs equal to or exceeding the global average value at that
epoch. Thus, this is an active period in their assembly. Based on their X-ray
luminosities and near-IR colors, as many as 25% of the massive galaxies at
z>1.5 host AGN, suggesting that the growth of supermassive black holes
coincides with massive-galaxy assembly.
