  Continuum polarization over the UV-to-microwave range is due to dichroic
extinction (or emission) by asymmetric, aligned dust grains. Because of both
grain alignment and scattering physics, the wavelength dependence of the
polarization, generally, traces the size of the aligned grains. Ultraviolet
(UV) polarimetry therefore provides a unique probe of the smallest dust grains
(diameter$<0.09\mu$m), their mineralogy and interaction with the environment.
However, the current observational status of interstellar UV polarization is
very poor with less than 30 lines of sight probed. With the modern,
quantitative and well-tested, theory of interstellar grain alignment now
available, we have the opportunity to advance the understanding of the
interstellar medium by executing a systematic study of the UV polarization in
the ISM of the Milky Way and near-by galaxies. The Polstar mission will provide
the sensitivity and observing time needed to carry out such a program,
addressing questions of dust composition as a function of size and location,
radiation- and magnetic-field characteristics as well as unveiling the carrier
of the 2175\AA\ extinction feature. In addition, using high-resolution UV line
spectroscopy Polstar will search for and probe the alignment of, and
polarization from, aligned atoms and ions - so called "Ground State Alignment",
a potentially powerful new probe of magnetic fields in the diffuse ISM.
