  Radio continuum emission at cm wavelengths is relatively little affected by
extinction. When combined with far-infrared (FIR) surveys this provides for a
convenient and unbiased method to select (radio-loud) AGN and starbursts deeply
embedded in gas and dust--rich galaxies. Such radio-selected FIR samples are
useful for detailed investigations of the complex relationships between (radio)
galaxy and starburst activity, and to determine whether ULIRGs are powered by
hidden quasars (monsters) or young stars (babies). We present the results of a
large program to obtain identifications and spectra of radio-selected,
optically faint IRAS/FSC objects using the FIRST/VLA 20 cm survey. These
objects are all radio-`quiet' in the sense that their radio power / FIR
luminosities follow the well-known radio/FIR relationship for star forming
galaxies. We compare these results to a previous study by our group of a sample
of radio-`loud' IRAS/FSC ULIRGs selected from the Texas 365 MHz survey. Many of
these objects also show evidence for dominant, A-type stellar populations, as
well as high ionization lines usually associated with AGN. These radio-loud
ULIRGs have properties intermediate between those of starbursts and quasars,
suggesting a possible evolutionary connection. Deep Keck spectroscopic
observations of three ULIRGs from these samples are presented, including high
signal-to-noise spectropolarimetry. The polarimetry observations failed to show
evidence of a hidden quasar in polarized (scattered) light in the two systems
in which the stellar light was dominated by A-type stars. Although observations
of a larger sample would be needed to allow a general conclusion, our current
data suggest that a large fraction of ULIRGs may be powered by luminous
starbursts, not by hidden, luminous AGN (quasars). [abridged]
