  Since the first discovery of microlensing events nearly two decades ago,
gravitational microlensing has accumulated tens of TBytes of data and developed
into a powerful astrophysical technique with diverse applications. The review
starts with a theoretical overview of the field and then proceeds to discuss
the scientific highlights. (1) Microlensing observations toward the Magellanic
Clouds rule out the Milky Way halo being dominated by MAssive Compact Halo
Objects (MACHOs). This confirms most dark matter is non-baryonic, consistent
with other observations. (2) Microlensing has discovered about 20 extrasolar
planets (16 published), including the first two Jupiter-Saturn like systems and
the only "cold Neptunes" yet detected. They probe a different part of the
parameter space and will likely provide the most stringent test of core
accretion theory of planet formation. (3) Microlensing provides a unique way to
measure the mass of isolated stars, including brown dwarfs to normal stars.
Half a dozen or so stellar mass black hole candidates have also been proposed.
(4) High-resolution, target-of-opportunity spectra of highly-magnified dwarf
stars provide intriguing "age" determinations which may either hint at enhanced
helium enrichment or unusual bulge formation theories. (5) Microlensing also
measured limb-darkening profiles for close to ten giant stars, which challenges
stellar atmosphere models. (6) Data from surveys also provide strong
constraints on the geometry and kinematics of the Milky Way bar (through proper
motions); the latter indicates predictions from current models appear to be too
anisotropic compared with observations. The future of microlensing is bright
given the new capabilities of current surveys and forthcoming new telescope
networks from the ground and from space. Some open issues in the field are
identified and briefly discussed.
