  The envelopes and disks that surround protostars reflect the initial
conditions of star and planet formation and govern the assembly of stellar
masses. Characterizing these structures requires observations that span the
near-infrared to centimeter wavelengths. Consequently, the past two decades
have seen progress driven by numerous advances in observational facilities
across this spectrum, including the \textit{Spitzer Space Telescope},
\textit{Herschel Space Observatory}, the Atacama Large Millimeter/submillimeter
Array, and a host of other ground-based interferometers and single-dish radio
telescopes. Nearly all protostars appear to have well-formed circumstellar
disks that are likely to be rotationally-supported; the ability to detect a
disk around a protostar is more a question of spatial resolution than whether
or not a disk is present. The disks around protostars have inherently higher
millimeter/submillimeter luminosities as compared to disks around more-evolved
pre-main sequence stars, though there may be systematic variations between star
forming regions. The envelopes around protostars are inherently asymmetric and
streamers emphasize that mass flow through the envelopes to the disks may not
be homogeneous. The current mass distribution of protostars may be impacted by
selection bias given that it is skewed toward solar-mass protostars,
inconsistent with the stellar initial mass function.
