  Observations at long wavelengths, in the wide interval from a few to 1000
micron, are essential to study diffuse media in galaxies, including all kinds
of atomic, ionic and molecular gases and dust grains. Hence they are
particularly suited to investigate the early phases in galaxy evolution, when a
very rich ISM is present in the forming systems.
  During the last few years a variety of observational campaigns in the
far-IR/sub-mm, exploiting both ground-based and space instrumentation, have
started to provide results of relevant cosmological impact. Most crucial among
these have been the discovery of an intense diffuse background in the
far-IR/sub-mm of extragalactic origin, and the deep explorations from space in
the far-IR and with large millimetric telescopes on ground. These results
challenge those obtained from optical-UV observations, by revealing luminous to
very luminous phases in galaxy evolution at substantial redshifts, likely
corresponding to violent events of star-formation in massive systems. This is
bringing to significant refinements of the present schemes of galaxy formation,
as far as the history of baryon transformations is concerned.
