Gamma-ray Emitting Narrow-Line Seyfert 1 Galaxies: Past, Present, and
  Future
  This article reviews our current understanding about $\gamma$-ray detected
narrow-line Seyfert 1 ($\gamma$-NLSy1) galaxies. The detection with the Large
Area Telescope onboard {\it Fermi}~Gamma-ray Space Telescope has provided the
strongest evidence for the presence of closely aligned relativistic jet in
these intriguing active galactic nuclei (AGN) and opened up a realm to explore
the physical conditions needed to launch the jet in a different central engine
and host galaxy environment than that is known for blazars. Promising results
acquired from various multi-wavelength campaigns are converging to a scenario
in which the $\gamma$-NLSy1 galaxies can be considered as `young' blazars.
These enigmatic sources hold the key to unravel the jet triggering mechanism
and evolution of the AGN phase of a galaxy, in general. As such,
$\gamma$-NLSy1s should be considered as one of the top priority targets for
next generation observational facilities.
