  We present constraints on the stellar-mass distribution of distant galaxies.
These stellar-mass estimates derive from fitting population-synthesis models to
the galaxies' observed multi-band spectrophotometry. We discuss the complex
uncertainties (both statistical and systematic) that are inherent to this
method, and offer future prospects to improve the constraints. Typical
uncertainties for galaxies at z ~ 2.5 are ~ 0.3 dex (statistical), and factors
of ~ 3 (systematic). By applying this method to a catalog of NICMOS-selected
galaxies in the Hubble Deep Field North, we generally find a lack of
high-redshift galaxies (z > 2) with masses comparable to those of present-day
``L*'' galaxies. At z < 1.8, galaxies with L*-sized masses do emerge, but with
a number density below that at the present epoch. Thus, it seems massive,
present-day galaxies were not fully assembled by z ~ 2.5, and that further star
formation and/or merging are required to assemble them from these high-redshift
progenitors. Future progress on this subject will greatly benefit from upcoming
surveys such as those planned with HST/ACS and SIRTF.
