  The review contains an analysis of the observed and model curves of the
interstellar extinction and polarization. The observations mainly give
information on dust in diffuse and translucent interstellar clouds. The
features of various dust grain models including spherical/non-spherical,
homogeneous/inhomogeneous particles are discussed. A special attention is
devoted to the analysis of the grain size distributions, alignment mechanisms
and magnetic field structure in interstellar clouds. It is concluded that the
interpretation of interstellar extinction and polarization is not yet complete.
