  (Abridged) High dispersion spectrographs on large aperture telescopes have
recently allowed observers to study the stellar and accretion properties of
deeply embedded young stars, commonly referred to as Class I stars. We
summarize these newly determined properties and compare them with observations
of more optically revealed Class II (T Tauri) stars. Class I stars have
spectral types and stellar luminosities similar to those of Class II stars,
suggesting similar masses and ages. Estimates of stellar luminosity and age,
however, are especially uncertain given the large extinctions, scattered light
emission and continuum excesses typical of Class I stars. Several candidate
Class I brown dwarfs are identified. Class I stars appear to rotate more
rapidly than T Tauri stars, by roughly a factor of 2. Likewise, Class I disk
accretion rates are only a factor of two larger than those of T Tauri stars,
less than the mass infall rates predicted by envelope models by 1-2 orders of
magnitude. In at least a few cases the discrepancy appears to be caused by T
Tauri stars being misclassified as Class I stars because of their edge-on disk
orientation. Stars where the envelope density and infall velocity have been
determined directly and unambiguously imply that stellar mass is not acquired
in a steady-state fashion, but instead through brief outbursts of enhanced
accretion. If some Class I stars are in fact as old as T Tauri stars,
replenishment may be necessary to sustain the long-lived envelopes, possibly
via continued dynamical interactions with cloud material.
