Properties of interstellar filaments as derived from $Herschel$,
  $Planck$, and molecular line observations
  Recent \herschel\ and \planck\ observations of submillimeter dust emission
revealed the omnipresence of filamentary structures in the interstellar medium
(ISM). The ubiquity of filaments in quiescent clouds as well as in star-forming
regions indicates that the formation of filamentary structures is a natural
product of the physics at play in the magnatized turbulent cold ISM.
  An analysis of more than 270 filaments observed with {\it Herschel} in 8
regions of the Gould Belt, shows that interstellar filaments are characterized
by a narrow distribution of central width, while they span a wide column
density range.
  Molecular line observations of a sample of these filaments show evidence of
an increase in the velocity dispersion of dense filaments with column density,
suggesting an evolution in mass per unit length due to accretion of surrounding
material onto these star-forming filaments.
  The analyses of \planck\ dust polarization observations show that the mean
magnetic field along the filaments is different from that of their surrounding
clouds. This points to a coupling between the matter and the $\vec{B}$-field in
the filament formation process. These observational results, derived from dust
and gas tracers in total and polarized intensity, set strong constraints on
theoretical models for filament formation and evolution. They also provide
important hints on the initial conditions of the star formation process from
the fragmentation of dense (supercritical) filaments. Higher resolution dust
polarization observations and large scale molecular line mapping are
nevertheless required to investigate in more details the internal structure of
interstellar filaments.
