  Primordial black holes (PBHs) may form in the early stages of the Universe
via the collapse of large density perturbations. Depending on the formation
mechanism, PBHs may exist and populate today the galactic halos and have masses
in a wide range, from about 10^{-14}Msun up to thousands, or more, of solar
masses. Gravitational microlensing is the most robust and powerful method to
constrain primordial black holes (PBHs), since it does not require that the
lensing objects be directly visible. We calculate the optical depth and the
rate of microlensing events caused by PBHs eventually distributed in the Milky
Way halo, towards some selected directions of observation. Then we discuss the
capability of Euclid, a space-based telescope which might perform microlensing
observations at the end of its nominal mission, to probe the PBH populations in
the Galactic halo.
