  In the currently-favored paradigm of planet formation, the location of the
snow line in the protoplanetary disk plays a crucial role. Determining the
demographics of planets beyond the snow line of stars of various masses is thus
essential for testing this model. Microlensing is sensitive to planets that are
generally inaccessible to other methods, and in particular is most sensitive to
cool planets at or beyond the snow line, including very low-mass (i.e.
terrestrial) planets. Hence, microlensing is uniquely suited and so essential
for a comprehensive study of this region. Microlensing is also sensitive to
planets orbiting low-mass stars, free-floating planets, planets in the Galactic
bulge and disk, and even planets in external galaxies. These planets can also
provide critical constraints on models of planet formation. Although
microlensing searches have so far detected only a handful of planets, these
have already changed our understanding of planet formation beyond the snow
line. Next generation microlensing surveys, which would be sensitive to tens of
"cold Earths" in this region, are well advanced in design conception and are
starting initial practical implementation.
