  Nanometer- and micrometer-sized solid particles play an important role in the
evolutionary cycle of stars and interstellar matter. The optical properties of
cosmic grains determine the interaction of the radiation field with the solids,
thereby regulating the temperature structure and spectral appearance of dusty
regions. Radiation pressure on dust grains and their collisions with the gas
atoms and molecules can drive powerful winds. The analysis of observed spectral
features, especially in the infrared wavelength range, provides important
information on grain size, composition and structure as well as temperature and
spatial distribution of the material.
  The relevant optical data for interstellar, circumstellar, and protoplanetary
grains can be obtained by measurements on cosmic dust analogs in the laboratory
or can be calculated from grain models based on optical constants. Both
approaches have made progress in the last years, triggered by the need to
interpret increasingly detailed high-quality astronomical observations. The
statistical theoretical approach, spectroscopic experiments at variable
temperature and absorption spectroscopy of aerosol particulates play an
important role for the successful application of the data in dust astrophysics.
