  This chapter introduces the concepts of polarimetry in the case of low
frequency radio astronomy. In this regime radio waves are usually not the
signature of atomic or molecular transitions lines, but rather that of unstable
particle distribution functions releasing their free energy through
electromagnetic radiation. As the radio source region is usually magnetized,
the propagation medium (at least close to the source) is anisotropic, and the
polarization depends on the local magnetic field direction, the propagation
mode and the direction of propagation.
