  Our current understanding of the physical processes of star formation is
reviewed, with emphasis on processes occurring in molecular clouds like those
observed nearby. The dense cores of these clouds are predicted to undergo
gravitational collapse characterized by the runaway growth of a central density
peak that evolves toward a singularity. As long as collapse can occur, rotation
and magnetic fields do not change this qualitative behavior. The result is that
a very small embryonic star or protostar forms and grows by accretion at a rate
that is initially high but declines with time as the surrounding envelope is
depleted. Rotation causes some of the remaining matter to form a disk around
the protostar, but accretion from protostellar disks is not well understood and
may be variable. Most, and possibly all, stars form in binary or multiple
systems in which gravitational interactions can play a role in redistributing
angular momentum and driving episodes of disk accretion. Variable accretion may
account for some peculiarities of young stars such as flareups and jet
production, and protostellar interactions in forming systems of stars will also
have important implications for planet formation. The most massive stars form
in the densest environments by processes that are not yet well understood but
may include violent interactions and mergers. The formation of the most massive
stars may have similarities to the formation and growth of massive black holes
in very dense environments.
