  Polarization of optical starlight and far-infrared thermal dust emission due
to alignment of interstellar grains offers a powerful window to study magnetic
fields in the various astrophysical environments, from the diffuse interstellar
medium to accretion disks surrounding young stars. Precision cosmology requires
accurate model of Galactic dust polarization for the first detection of Cosmic
Microwave Background (CMB) B-modes signal. Such an accurate model is only
achieved when a quantitative theory of grain alignment that links grain
alignment efficiency with local physical conditions and dust properties is
developed and tested. In this paper, we review the successful development of
such a quantitative alignment theory, focusing on a unified model of grain
alignment based on radiative torques and magnetic relaxation for dust grains
incorporated with iron inclusions. We discuss the implication of the unified
alignment model for interpreting the latest observational results by Planck and
ALMA.
