The Evolution of Protostars: Insights from Ten Years of Infrared Surveys
  with Spitzer and Herschel
  Stars form from the gravitational collapse of dense molecular cloud cores. In
the protostellar phase, mass accretes from the core onto a protostar, likely
through an accretion disk, and it is during this phase that the initial masses
of stars and the initial conditions for planet formation are set. Over the past
decade, new observational capabilities provided by the Spitzer Space Telescope
and Herschel Space Observatory have enabled wide-field surveys of entire
star-forming clouds with unprecedented sensitivity, resolution, and infrared
wavelength coverage. We review resulting advances in the field, focusing both
on the observations themselves and the constraints they place on theoretical
models of star formation and protostellar evolution. We also emphasize open
questions and outline new directions needed to further advance the field.
