  Here we review some of the main issues related to multi-wavelength source
identification and characterization, with particular emphasis on the field of
X-ray surveys carried out over the last years. This complex and time-consuming
process is going to represent one of the main difficulties over the coming
years, when significantly larger surveys, both in area and depth, will be
carried out with the new generations of space- and ground-based facilities like
e.g. eROSITA, WISE, VISTA, Pan-STARRS, and LSST. The Virtual Observatory can
offer a reliable way to approach to a new concept of data handling and
multi-wavelength source characterization, provided that uniform and rigorous
data analyses and extensive quality checks are performed.
