  Infrared photometry and later infrared spectroscopy provided powerful
diagnostics to distinguish between the main emission mechanisms in galaxies:
AGN and Starburst. After the pioneering work on infrared photometry with IRAS
in the far-IR and the S.Pedro Martir and ESO ground-based work in the near-IR,
ISO photometry extended up to 200um the coverage of the galaxies energy
distributions. Then Spitzer collected accurate mid-infrared spectroscopy on
different samples of galaxies. We will review the work done on the 12um galaxy
sample since the times of IRAS photometry to the new Spitzer spectroscopy. The
main results on the multifrequency data of 12um selected Seyfert galaxies are
presented and discussed in the light of unification and evolution models. The
spectroscopic work of Spitzer will soon be complemented at longer wavelengths
by the Herschel spectrometers and in the future by SPICA at higher redshift.
