  Present AGN and starburst models aiming to account for the observed infrared
SEDs consider a physical description of the dust and a solution of the
radiative transfer problem. MIR spectra obtained at different spatial scales
(SST-IRS, ISO and TIMMI2) are presented. They show that PAH bands are detected
in starburst regions but significantly reduced near the centre of AGN. This is
explained by examining the heating mechanism of PAHs after hard (FUV, X-ray)
photon interactions. Economic radiative transfer models of starbursts and AGN
are made available. The successful application of the starburst model is
demonstrated by fitting broad band data and detailed Spitzer spectra of
NGC7714. The AGN model is applied to ISO data of a sample of 68 radio galaxies
and quasars of the 3CR catalogue. Radiative transfer models of galaxies with
Hidden Broad Line Regions are shown. Their SED enable us to separate the
contributions from the dusty disc of the AGN and the starbursts. The composite
model is consistent with the unified scheme and the idea that the infrared
emission of AGN is dominated in the MIR and starbursts in the FIR. According to
the unified scheme, the observed diversity of AGN properties results from the
different orientations relative to our line of sight. The strong resonance of
silicate dust at 10mic is therefore, as expected, seen in absorption towards
many type-2 AGN. In type-1 AGN, it should be seen in emission because the hot
inner surface of the dust torus becomes visible. However, this has not been
observed so far, thus challenging the unification scheme or leading to exotic
modifications of the dust-torus model. Here the recent discovery of the 10mic
silicate feature in emission in luminous quasar spectra is reported.
