On the 511 keV emission line of positron annihilation in the Milky Way

I review our current understanding of positron sources in the Galaxy, on the basis of the reported properties of the observed 511 keV annihilation line. It is argued here that most of the disk positrons propagate away from the disk and the resulting low surface brightness annihilation emission is currently undetectable by SPI/INTEGRAL. It is also argued that a large fraction of the disk positrons may be transported via the regular magnetic field of the Galaxy into the bulge and annihilate there. These ideas may alleviate current difficulties in interepreting INTEGRAL results in a "conventional" framework.