Starting with the early experiment of observing polarization of radiation from canal rays in an electric field, we review spatially anisotropic excitation processes which create aligned atoms or ions, leading to polarization of the emitted radiation by these atoms or ions. Collisional relaxation of the alignment is also reviewed. A formulation of the alignment collisional‐radiative model is presented for treating polarization of radiation emitted from an ensemble of atoms in plasma under an axially symmetric condition. A preliminary result is given for helium atoms in a beam injected into a plasma