  [abridged] At the end of the Spitzer cryogenic mission and the onset of the
Herschel era, we review our current knowledge on galaxy evolution at IR
wavelengths. We also develop new tools for the analysis of background
fluctuations to constrain source counts in regimes of high confusion. We
analyse a large variety of new data on galaxy evolution and high-z source
populations from Spitzer surveys, as well as complementary data from sub-mm
(BLAST) and millimetric ground-based observations. These data confirm earlier
indications about a very rapid increase of galaxy volume emissivity up to z~1.
This is the fastest evolution rate observed for galaxies at any wavelengths.
The observed Spitzer counts at 24 micron require a combination of fast
evolution for the dominant population and a bumpy spectrum with substantial PAH
emission at z~1 to 2. Confusion-limited number counts at longer wavelengths
confirm these results. All the present data require that the fast observed
evolution from z=0 to 1 flattens around redshift 1 and then keeps approximately
constant up to z~2.5 at least. The present-day IR data provide clear evidence
for the existence of a distinct population of very luminous galaxies becoming
dominant at z > 1. Their cosmological evolution, peaking around z~2, shows a
faster decay with cosmic time than lower luminosity systems, whose maximal
activity is set around z~1, then supporting an earlier phase of formation for
the most luminous and massive galaxies. From a comparison of our results on the
comoving IR emissivity with recent estimates of the redshift-dependent stellar
mass functions of galaxies, we infer that a large fraction (>=50%) of the IR
activity at z > 1 should be due to obscured AGN accretion and that concomitant
SF in high-z luminous sources should follow a top-heavy IMF.
