  Aligned grains provide one of the easiest ways to study magnetic fields in
diffuse gas and molecular clouds. How reliable our conclusions about the
inferred magnetic field depends critically on our understanding of the physics
of grain alignment. Although grain alignment is a problem of half a century
standing recent progress achieved in the field makes us believe that we are
approaching the solution of this mystery. I review basic physical processes
involved in grain alignment and show why mechanisms that were favored for
decades do not look so promising right now. I also discuss why the radiative
torque mechanism ignored for more than 20 years looks right now the most
powerful means of grain alignment.
