  Observations of the stellar initial mass function are reviewed. The
flattening at low mass is evidence for a characteristic mass in star formation,
which could be the minimum stellar mass for the onset of deuterium burning or
the thermal Jeans mass. Variations in the mass of the flattened part are
important for understanding star formation; e.g., starburst galaxies may be an
example where the characteristic mass is larger than it is locally. A steeper
high-mass slope in the extreme field studied by Massey et al. may be the result
of cloud destruction and the termination of star formation by ionization, with
a normal IMF in each separate cluster. The lack of a density dependence in
cluster IMFs suggests that star and protostar interactions play little role in
star formation or the IMF. These considerations, along with indirect
observations of the IMF in the early Universe, suggest that the IMF does not
vary much in its basic form over position and time, but that shifts in the
characteristic mass might occur in regions with extremely high or low star
formation activity.
