  Galactic HI is a gas that is coupled to magnetic field because of its
fractional ionization. Many properties of HI are affected by turbulence.
Recently, there has been a significant breakthrough on the theory of
magnetohydrodynamic (MHD) turbulence. For the first time in the history of the
subject we have a scaling model that is supported by numerical simulations. We
review recent progress in studies of both incompressible and compressible
turbulence. We also discuss the new regime of MHD turbulence that happens below
the scale at which conventional turbulent motions get damped by viscosity. The
viscosity in the case of HI is being produced by neutrals and truncates the
turbulent cascade at parsec scales. We show that below this scale magnetic
fluctuations with a shallow spectrum persist and point out to a possibility of
the resumption of the MHD cascade after ions and neutrals decouple. We discuss
the implications of the new insight into MHD turbulence for cosmic ray
transport and grain dynamics.
