  Infrared surveys provide essential insights on galaxy evolution. If near-IR
studies suggest mild evolution of stellar populations with cosmic time,
indications of a substantial evolution have been seen in the far-IR, although
the available information is largely insufficient to delineate precise
evolutionary properties. A consistent picture encompassing all the currently
available data may be obtained assuming that dust extinction hides the early
evolutionary phases of spheroidal galaxies in the optical band, while the
corresponding dust re-radiation in the far-IR may have been, during the early
evolutionary phases, orders of magnitude larger than today. Hyperluminous IRAS
galaxies might be extreme examples of this situation. Additional indications
that at least some spheroidal galaxies may have been very dusty during their
early evolution are provided by recent data on high redshift radio galaxies and
quasars. Galaxies are the likely dominant contributors to the IR background.
However, in the framework of unified models for Active Galactic Nuclei, a large
number of nuclei hidden by a dusty torus may be expected. Implications for the
IR background are discussed.
