Polarization is a basic property of light and is fundamentally linked to the internal geometry of a source of radiation. Polarimetry complements photometric, spectroscopic, and imaging analyses of sources of radiation and has made possible multiple astrophysical discoveries. In this article I review (i) the physical basics of polarization: electromagnetic waves, photons, and parameterizations; (ii) astrophysical sources of polarization: scattering, synchrotron radiation, active media, and the Zeeman, Goldreich- Kylafis, and Hanle effects, as well as interactions between polarization and matter (like birefringence, Faraday rotation, or the Chandrasekhar-Fermi effect); (iii) observational methodology: on-sky geometry, influence of atmosphere and instrumental polarization, polarization statistics, and observational techniques for radio, optical, and X/γ wavelengths; and (iv) science cases for astronomical polarimetry: solar and stellar physics, planetary system bodies, interstellar matter, astrobiology, astronomical masers, pulsars, galactic magnetic fields, gamma-ray bursts, active galactic nuclei, and cosmic microwave background radiation.

ABSTRACT
1. INTRODUCTION
2. PHYSICAL BASICS
    2.1 Electromagnetic Waves
    2.2 Photons
    2.3 Parameterizations
3. POLARIGENESIS
    3.1 Scattering Polarization
    3.2 Dichroic Media
    3.3 Optically Active Media
    3.4 Synchrotron Radiation
    3.5 Zeeman Effect
    3.6 Goldreich–Kylafis Effect
    3.7 Hanle Effect
    3.8 Interactions of Polarization and Matter
4. OBSERVATIONS
    4.1 Sky Projection
    4.2 Terrestrial Atmosphere
    4.3 Instrumental Polarization
    4.4 Polarization Statistics
    4.5 Radio Observations
    4.6 Optical Observations
    4.7 X and γ Ray Observations
5. SCIENCE CASES
    5.1 Solar and Stellar Physics
    5.2 Planetary System Bodies
    5.3 Interstellar Matter
    5.4 Astrobiology
    5.5 Astronomical Masers
    5.6 Pulsars
    5.7 Active Galactic Nuclei
    5.8 Galactic Magnetic Fields
    5.9 Gamma Ray Bursts
    5.10 Cosmic Background Radiation
6. CONCLUSIONS
REFERENCES

• SASCHA TRIPPE(Department of Physics and Astronomy, Seoul National University)