  The 511 keV line from positron annihilation in the Galaxy was the first
$\gamma$-ray line detected to originate from outside our solar system. Going
into the fifth decade since the discovery, the source of positrons is still
unconfirmed and remains one of the enduring mysteries in $\gamma$-ray
astronomy. With a large flux of $\sim$10$^{-3}$ $\gamma$/cm$^{2}$/s, after 15
years in operation INTEGRAL/SPI has detected the 511 keV line at $>50\sigma$
and has performed high-resolution spectral studies which conclude that Galactic
positrons predominantly annihilate at low energies in warm phases of the
interstellar medium. The results from imaging are less certain, but show a
spatial distribution with a strong concentration in the center of the Galaxy.
The observed emission from the Galactic disk has low surface brightness and the
scale height is poorly constrained, therefore, the shear number of annihilating
positrons in our Galaxy is still not well know. Positrons produced in
$\beta^+$-decay of nucleosynthesis products, such as $^{26}$Al, can account for
some of the annihilation emission in the disk, but the observed spatial
distribution, in particular the excess in the Galactic bulge, remains difficult
to explain. Additionally, one of the largest uncertainties in these studies is
the unknown distance that positrons propagate before annihilation. In this
paper, we will summarize the current knowledge base of Galactic positrons, and
discuss how next-generation instruments could finally provide the answers.
