  The second Gaia data release (DR2), contains very precise astrometric and
photometric properties for more than one billion sources, astrophysical
parameters for dozens of millions, radial velocities for millions, variability
information for half a million of stellar sources and orbits for thousands of
solar system objects. Before the Catalogue publication, these data have
undergone dedicated validation processes. The goal of this paper is to describe
the validation results in terms of completeness, accuracy and precision of the
various Gaia DR2 data. The validation processes include a systematic analysis
of the Catalogue content to detect anomalies, either individual errors or
statistical properties, using statistical analysis, and comparisons to external
data or to models. Although the astrometric, photometric and spectroscopic data
are of unprecedented quality and quantity, it is shown that the data cannot be
used without a dedicated attention to the limitations described here, in the
Catalogue documentation and in accompanying papers. A particular emphasis is
put on the caveats for the statistical use of the data in scientific
exploitation.
