  This work reviews our understanding of the magnetic fields observed in the
quiet Sun. The subject has undergone a major change during the last decade
(quiet revolution), and it will remain changing since the techniques of
diagnostic employed so far are known to be severely biased. Keeping these
caveats in mind, our work covers the main observational properties of the quiet
Sun magnetic fields: magnetic field strengths, unsigned magnetic flux
densities, magnetic field inclinations, as well as the temporal evolution on
short time-scales (loop emergence), and long time-scales (solar cycle). We also
summarize the main theoretical ideas put forward to explain the origin of the
quiet Sun magnetism. A final prospective section points out various areas of
solar physics where the quiet Sun magnetism may have an important physical role
to play (chromospheric and coronal structure, solar wind acceleration, and
solar elemental abundances).
