  The current status of both the observational evidence and the theory of the
stellar initial mass function (IMF) is reviewed, with particular attention to
the two basic, apparently universal features shown by all observations of
nearby stellar systems: (1) a characteristic stellar mass of the order of one
solar mass, and (2) a power-law decline of the IMF at large masses similar to
the original Salpeter law. Considerable evidence and theoretical work supports
the hypothesis that the characteristic stellar mass derives from a
characteristic scale of fragmentation in star-forming clouds which is
essentially the Jeans scale as calculated from the typical temperature and
pressure in molecular clouds. The power-law decline of the IMF at large masses
suggests that the most massive stars are built up by scale-free accretion or
accumulation processes, and the observed formation of these stars in dense
clusters and close multiple systems suggests that interactions between dense
prestellar clumps or protostars in forming clusters will play a role. A simple
model postulating successive mergers of subsystems in a forming cluster
accompanied by the accretion of a fraction of the residual gas by the most
massive protostar during each merger predicts an upper IMF of power-law form
and reproduces the Salpeter law with a plausible assumed accretion efficiency.
