Gravitational Microlensing: Past, Present and Future

Ongoing microlensing surveys have already yielded more than five hundred microlensing events, most of which have been identified in real-time. In this review I present the basic theory and observational status of these surveys. I highlight the discoveries made so far - these include constraints on the dark matter content of the Galaxy, the structure and mass function of stars in the Galactic bulge and applications on stellar atmospheres and populations. I also briefly discuss the scientific returns (such as planet detection) from real-time photometric and spectroscopic follow-up surveys. With improved software algorithms and more powerful instrumentations, the number of microlensing events will be greatly increased in the future; the Space Interferometry Mission, on the other hand, provides the possibility to break the lens mass and distance degeneracy. I conclude that microlensing is developing into an exciting technique with diverse applications.