  Traditional approaches to measuring the stellar mass function (MF) are
fundamentally limited because objects are detected based on their luminosity,
not their mass. These methods are thereby restricted to luminous and relatively
nearby stellar populations. Gravitational microlensing promises to
revolutionize our understanding of the MF. It is already technologically
feasible to measure the MFs of the Galactic disk and Galactic bulge as
functions of position, although the actual execution of this program requires
aggressive ground-based observations including infrared interferometry, as well
as the launching of a small satellite telescope. Rapid developments in
microlensing, including the new technique of ``pixel lensing'' of unresolved
stars, will allow one to probe the MF and luminosity function of nearby
galaxies. Such observations of M31 are already underway, and pixel-lensing
observations of M87 with the {\it Hubble Space Telescope} would permit
detection of dark intra-cluster objects in Virgo. Microlensing techniques can
also be applied to investigate the star-formation history of the universe and
to search for planets with masses as small as the Earth's. Based on an invited
talk at the January 1996 AAS meeting in San Antonio. PASP (June 1996) in press,
(c) ASP, reproduced with permission.
