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One of the premier unsolved questions in cosmology is how massive galaxies
assembled their stars and central black holes. Multiwavelength observations
are now enabling us to address these questions directly by studying massive
galaxies during their main formation epoch z = 1.5--3. Deep infrared imaging
from Spitzer is playing a fundamental role in these studies. I will discuss
the constraints placed by IRAC, MIPS, and X-ray observations on the stellar
populations and star formation/AGN activity in this crucial redshift range.
A particular striking result is the photometric identification of substantial
numbers massive old "dead" galaxies at z>2 with very low specific star
formation rates, which appear to make up 30-40% of z~2.5 galaxies at the
high mass end. The first follow-up studies with NICMOS show that these old
massive galaxies are remarkably compact, presenting us with new puzzles
about their structural evolution to the present day. Pushing to the highest
redshifts, the unprecedented sensitivity of Spitzer/IRAC has allowed us
to probe the stellar-masses and ages of luminous galaxies at z > 7 in the
Ultra Deep Field, providing a glimpse of the earliest phases of galaxy
formation. I will place these results in the context of formation scenarios
for massive galaxies, and look ahead to the future with the expected launch
of the far-IR Herschel Telescope, and the installation of WFC3 on Hubble in
the next 18 months.
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