  We present high resolution (R = 18,000), high signal-to-noise, 2 micron
spectra of 52 infrared-selected Class I and flat-spectrum young stellar objects
in the Taurus-Auriga, $\rho$ Ophiuchi, Serpens, Perseus, and Corona Australis
dark clouds. We detect key absorption lines in 41 objects and fit synthetic
spectra generated from pre-main sequence models to deduce the effective
temperatures, surface gravities, near-infrared veilings, rotation velocities,
and radial velocities of each of these 41 sources. We find these objects to
span ranges in effective temperature, surface gravity, and stellar luminosity
which appear similar to those of late spectral-type Class II sources and
classical T-Tauri stars. We determine that the mean 2 micron veiling of Class I
and flat-spectrum objects is significantly higher than that of Class II objects
in the same region where both types of objects are extensively observed ($\rho$
Oph). We find a significant fraction of our protostellar sample also exhibits
emission lines. Twenty-three objects show H$_2$ emission, usually indicative of
the presence of energetic outflows. Thirty-four sources show HI Br $\gamma$
emission and a number of these exhibit profile asymmetries consistent with
in-fall. Eight sources show significant $\Delta v = 2$ CO emission suggestive
of emission from a circumstellar disk. Overall, these observations indicate
that Class I and flat-spectrum objects are self-embedded protostars undergoing
significant mass accretion, although the objects appear to span a broad range
of mass accretion activity.
