  In this review, we present a comprehensive overview of superconductivity in
electron-doped metal nitride halides $M$N$X$ ($M$ = Ti, Zr, Hf; $X$ = Cl, Br,
I) with layered crystal structure and two-dimensional electronic states. The
parent compounds are band insulators with no discernible long-range ordered
state. Upon doping tiny amount of electrons, superconductivity emerges with
several anomalous features beyond the conventional electron-phonon mechanism,
which stimulate theoretical investigations. We will discuss experimental and
theoretical results reported thus far and compare the electron-doped layered
nitride superconductors with other superconductors.
