  The most recent all-sky surveys performed with the INTEGRAL and SWIFT
satellites allowed the detection of more than 1500 sources in hard X-rays above
20 keV. About one quarter of them has no obvious counterpart at other
wavelengths and therefore could not be associated with any known class of
high-energy emitting objects. Although cross-correlation with catalogues or
surveys at other wavelengths (especially soft X-rays) is of invaluable support
in pinpointing the putative optical candidates, only accurate optical
spectroscopy can reveal the true nature of these sources. With the aim of
identifying them, we started in 2004 an optical spectroscopy program which uses
data from several telescopes worldwide and which proved extremely successful,
leading to the identification of about 200 INTEGRAL objects and nearly 130
Swift sources. Here we want to present a summary of this identification work
and an outlook of our preliminary results on identification of newly-discovered
sources belonging to the 4th IBIS catalog and to the 54 months Palermo
Swift/BAT catalog.
