  Polarimetry is one of the most informative techniques of studying magnetic
fields in molecular clouds. How reliable the interpretation of the polarization
maps in terms of magnetic fields is the issue that the grain alignment theory
addresses.
  We show that grain alignment involves several processes acting
simultaneously, but on different time-scales. We explain that rotating dust
grains get substantial magnetic moment that allows them precess fast about
magnetic field lines. As the result, grains preserve their orientation to
magnetic field when the magnetic field direction fluctuates. We point out to
the importance of internal alignment, i.e. the process forces grain axes to be
aligned in respect to the grain angular momentum. We show that subtle quantum
effects, in particular relaxation related to nuclear magnetic moments of atoms
composing the grain, brings to live complex grain motions, e.g. flips. These
flips substantially alter the dynamics of grain and limit the applicability of
earlier theories that did not account for them. We also briefly review basic
physical processes involved in the alignment of grain angular momentum in
respect to interstellar magnetic field. We claim that the bulk of existing
observational data is consistent with the radiative torque alignment mechanism.
In particular, we show that large grains that are known to exist in the cores
of molecular clouds may be aligned by the attenuated external interstellar
radiation field.
