  A proper analysis of the evolution of sources emitting in the Mid-Infrared is
strongly dependent on their broad-band spectral properties (SEDs) at different
redshifts and luminosities and on a reliable classification allowing to
disentangle AGN from star-formation activity. The diagnostic diagrams based on
the optical line ratios are often ambiguous and/or misleading not allowing a
proper separation of the galaxy/AGN populations. Thanks to the combination of
deep Spitzer and X-rays data a much better census of the hidden AGN activity
and dust-obscured star-forming galaxies can be obtained, constraining galaxy
and AGN evolutionary models.
