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.

Accepted for publication in Journal of Quantitative Spectroscopy and Radiative Transfer (Special issue, Electromagnetic and Light Scattering XIII, Taormina, 2011), 48 pages, 14 figures