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.
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