The distribution of stellar masses that form in one star-formation
event in a given volume of space is called the initial mass function
(IMF). The IMF has been estimated from low-mass brown dwarfs to very
massive stars. Combining IMF estimates for different populations in
which the stars can be observed individually unveils an extraordinary
uniformity of the IMF. This general insight appears to hold for
populations including present-day star formation in small molecular
clouds, rich and dense massive star-clusters forming in giant clouds,
through to ancient and metal-poor exotic stellar populations that may
be dominated by dark matter. This apparent universality of the IMF is
a challenge for star formation theory because elementary
considerations suggest that the IMF ought to systematically vary with
star-forming conditions.