  Luminous submillimeter galaxies (SMGs) have been known for some time to
produce a substantial fraction of the far-IR/submillimeter background. New
observations show that these systems are not only very luminous, but also very
massive. Here we present results from (a) a survey of SMGs' molecular line
emission, which has confirmed Keck optical redshifts and revealed their high
gas and dynamical masses; and (b) a detailed case study of the rest-frame
optical emission from one SMG, which has revealed its high metallicity and old
stellar population. We infer a high z~2.7 comoving number density of galaxies
with baryonic masses greater than 10^11 solar masses; this is not accounted for
in current semi-analytic models of galaxy evolution.
