Statistics of Centroids of Velocity
  We review the use of velocity centroids statistics to recover information of
interstellar turbulence from observations. Velocity centroids have been used
for a long time now to retrieve information about the scaling properties of the
turbulent velocity field in the interstellar medium. We show that, while they
are useful to study subsonic turbulence, they do not trace the statistics of
velocity in supersonic turbulence, because they are highly influenced by
fluctuations of density. We show also that for sub-Alfv\'enic turbulence (both
supersonic and subsonic) two-point statistics (e.g. correlation functions or
power-spectra) are anisotropic. This anisotropy can be used to determine the
direction of the mean magnetic field projected in the plane of the sky.
