  Low-mass protostars are less luminous than expected. This luminosity problem
is important because the observations appear to be inconsistent with some of
the basic premises of star formation theory. Two possible solutions are that
stars form slowly, which is supported by recent data, and/or that protostellar
accretion is episodic; current data suggest that the latter accounts for less
than half the missing luminosity. The solution to the luminosity problem bears
directly on the fundamental problem of the time required to form a low-mass
star. The protostellar mass and luminosity functions provide powerful tools
both for addressing the luminosity problem and for testing theories of star
formation. Results are presented for the collapse of singular isothermal
spheres, for the collapse of turbulent cores, and for competitive accretion.
