(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.
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