The 511 keV-line emission from positron annihilation is the first gamma-ray line ever detected outside the solar system. Despite 30 years of intense theoretical and observational investigation, the sources of positrons and their fate until annihilation remain unknown at present. ESA’s INTEGRAL gamma-ray observatory with the SPI instrument allowed for the first time to map that emission. This revealed an unexpected morphology for the gamma-ray emission: It is strongly concentrated in/around the Galactic bulge, emission from the disk of the Galaxy is much weaker than expected. The bulge/disk luminosity ratio is much larger than observed in any other wavelength. This mapping, though preliminary, prompted a number of novel explanations, including rather “exotic” ones (like e.g. dark matter annihilation). However, the conventional sources, like e.g. type Ia supernovae, microquasars or X-ray binaries, are still plausible explanations of the observed total gamma-ray intensity. A closer study of the subject reveals new layers of complexity, since positrons (even of low energy, like those produced by radioactivity in supernovae) may propagate far away from their sources, so that inference of the underlying source distribution from the observed gamma-ray image is less direct. For those reasons, the 511 keV line of the Milky Way is undoubtedly the hottest subject in gamma-ray line astronomy today.

• Sources of positrons (positron yields and Galaxy wide source distribution)
• Propagation of positrons (slowing down and thermalization in various media and propagation in the Galactic magnetic field)
• Annihilation of positrons (resulting gamma-ray signatures as a function of the physical conditions in the annihilation site)
• The annihilation gamma-ray sky (assessment of spatial morphology and spectroscopy of the emission; interpretations)