We present the results of an on-going program for the identification and characterization of optical counterparts of Ultra-Luminous X-ray (ULX) sources. The targets have been selected from the catalogues by Colbert & Ptak (2002) and Swartz et al. (2004). A clear identification based on unambiguous optical spectral features was possible for 26 objects. A large number of objects result to be QSOs at higher redshift than the putative parent galaxy, and other ULXs seem to be associated to HII regions. In a few cases the optical counterpart results a foreground star in our galaxy. The observational program will continue to obtain a representative sample for statistical studies.