  Gravitational microlensing is a robust tool to detect and directly measure
the abundance and mass of any kind of compact objects, either in our galaxy or
in the extragalatic domain. On basis to generic, broadly applicable arguments,
it is concluded that the observed microlensing magnifications are too small and
the microlensing events less frequent than the expectations for a significant
population of compact objects (other than normal stars). The detection of
chromatic effects of microlensing, neither supports the presence of BHs.
Detailed statistical studies of the observed microlensing magnifications and
events frequency impose strict upper limits to the fraction of total mass of
BHs ($\ltsim$ 1\%) from $10^{-7}M_\odot$ to indefinitely large masses. These
results hold even when the BHs are distributed according to a mass spectrum or
are forming clusters.
