  Four planets have recently been discovered by gravitational microlensing. The
most recent of these discoveries is the lowest-mass planet known to exist
around a normal star. The detection of planets in gravitational microlensing
events was predicted over a decade ago. Microlensing is now a mature field of
astrophysical research and the recent planet detections herald a new chapter in
the hunt for low mass extra-solar planets. This paper reviews the basic theory
of planetary microlensing, describes the experiments currently in operation for
the detection and observation of microlensing events and compares the
characteristics of the planetary systems found to date by microlensing. Some
proposed schemes for improving the detection rate of planets via microlensing
are also discussed.
