  Current limits on the intensity of the extragalactic infrared background are
consistent with the expected contribution from evolving galaxies. Depending on
the behaviour of the star formation rate and of the initial mass function, we
can expect that dust extinction during early evolutionary phases ranges from
moderate to strong. An example of the latter case may be the ultraluminous
galaxy IRAS F$10214 + 4724$. The remarkable lack of high redshift galaxies in
faint optically selected samples may be indirect evidence that strong
extinction is common during early phases. Testable implications of different
scenarios are discussed; ISO can play a key role in this context. Estimates of
possible contributions of galaxies to the background under different
assumptions are presented. The COBE/FIRAS limits on deviations from a blackbody
spectrum at sub-mm wavelengths already set important constraints on the
evolution of the far-IR emission of galaxies and on the density of obscured
(``Type 2'') AGNs. A major progress in the field is expected at the completion
of the analysis of COBE/DIRBE data.
