  Several lines of evidence and theoretical arguments suggest that a large
fraction of starlight is absorbed by interstellar dust and re-radiated at
far-IR wavelengths, particularly during early evolutionary phases of early type
galaxies, which may even, under some circumstances, experience an optically
thick phase. Therefore far-IR to mm observations are crucial to understand the
galaxy evolution. The strong K-correction makes surveys at mm wavelengths
ideally suited for studying high-$z$ galaxies. The broad redshift range covered
by mm surveys at sub-mJy flux limits offers a good chance for gaining important
information also on the geometry of the Universe.
