  Our present understanding of the extragalactic source counts and background
radiation at infrared and sub-mm wavelengths is reviewed. Available count data
are used to constrain evolutionary models of galaxies and Active Nuclei. The
CIRB, on the other hand, provides crucial information on the integrated past IR
emissivity, including sources so faint to be never accessible. The near-IR and
sub-mm are suited to search for the CIRB. These cosmological windows are ideal
to detect redshifted photons from the two most prominent broad emission
features in galaxy spectra: the stellar photospheric 1 micron peak and the one
at 100 micron due to dust re-radiation. The recently claimed detection of an
isotropic diffuse component at 100-200 microns would support the evidence for
strong cosmic evolution from $z=0$ to $z\sim 2$ of faint gas-rich late-type
galaxies, as inferred from direct long-$\lambda$ counts. Furthermore, an
equally intense CIRB flux measured in the wavelength range 200 to 500 $\mu m$
may be in support of models envisaging a dust-enshrouded phase during formation
of spheroidal galaxies (e.g. Franceschini et al., 1994): the background
spectrum at such long wavelengths would indicate an high redshift ($z>2-3$) for
this active phase of star formation, implying a relevant constraint on
structure formation scenarios. We argue that, given the difficulties for sky
surveys in this spectral domain, only sporadic tests of these ideas will be
achieved in the next several years. Only a mission like FIRST, in combination
with large mm arrays and optical telescopes, will allow to thoroughly
investigate, via photometric and spectroscopic surveys, these IR-bright early
phases in the evolution of galaxies and AGNs.
