  Understanding the formation history of massive galaxies is one of most
popular and longstanding problems in astronomy, with observations and theory
addressing how and when these systems assembled. Since the most massive
galaxies in today's universe, with M_*> 10^11 M_0, are nearly all elliptical
with uniform old stellar populations, we must probe higher redshifts to
discover their full origins. A recent consensus has developed that nearly all
M_* > 10^11 M_0 galaxies we see today were established by z~1, with at most a
factor of two growth in stellar mass and number densities at lower redshifts.
We review the evidence for this, and discuss how recent observations of star
formation rates, colors, and morphologies of massive galaxies at z < 1 with M_*
> 10^11 M_0 show that these systems are still experiencing some evolution.
Massive galaxies undergo on average a single major merger at z < 1.5, and
roughly half are experiencing star formation at the same redshifts. The highest
mass galaxies, with M_* > 10^11.5 M_0, appear in similar abundance at z < 2,
suggesting that extremely massive galaxies are mostly formed very early in the
universe. Observations at z > 1.5 demonstrate that major galaxy mergers are the
primary method for assembling these massive galaxies, with nearly all of this
merging occurring at z > 2, with on average 4 to 5 major mergers taking place
at z = 1.5 - 3.
