In the previous chapters, we have seen how a large mass distribution, like a galaxy, can act as a gravitational lens. But smaller masses, such as stars or compact objects (e.g. black holes, brown dwarfs, planets, etc.), can also act as lenses. The deflection of light and hence, the angular separation between the lensed images of a background source are proportional to the square-root of the mass of the lens. For stellar mass objects, this implies that the image separation is typically of the order of one milli-or micro-arcsecond, hence the term microlensing introduced by Paczyński (1986a). Such small angular separations can not be resolved with current instruments, and the micro-images form one single unresolved (de-)magnified image. The only observable effect of microlensing is the global (de-)magnification of the lensed source, which can reach several magnitudes. A very interesting property of microlensing is that it is a dynamical phenomenon. The relative motions between the observer, the source, and the microlenses induce flux variations on time-scales of a few days to several months depending on the lensed system. These evolving brightness fluctuations are called microlensing events.