  It has been shown by Paczy\'nski that gravitational microlensing is
potentially a useful method for detecting the dark constituents of the halo of
our galaxy, if their mass lies in the approximate domain
$10^{-6}<M/M_\odot<10^{-1}$. Microlensing observations now im progress monitor
several millions of stars in the Large Magellanic Cloud and in the Galactic
Bulge. Here I discuss the main features of the microlensing events: in
particular their rates and probability.
