  Neutron star mergers have been long considered as promising sites of heavy
$r$-process nucleosynthesis. We overview observational evidence supporting this
scenario including: the total amount of $r$-process elements in the Galaxy,
extreme metal poor stars, geological radioactive elemental abundances, dwarf
galaxies, and short gamma-ray bursts (sGRBs). Recently, the advanced LIGO and
Virgo observatories discovered a gravitational-wave signal of a neutron star
merger, GW170817, as well as accompanying multi-wavelength electromagnetic (EM)
counterparts. The ultra-violet, optical, and near infrared observations point
to $r$-process elements that have been synthesized in the merger ejecta. The
rate and ejected mass inferred from GW170817 and the EM counterparts are
consistent with other observations. We find however that, within simple one
zone chemical evolution models (based on merger rates with reasonable delay
time distributions as expected from evolutionary models, or from observations
of sGRBs), it is difficult to reconcile the current observations of the
europium abundance history of Galactic stars for [Fe/H] $\gtrsim -1$. This
implies that to account for the role of mergers in the Galactic chemical
evolution, we need a Galactic model with multiple populations that have
different spatial distributions and/or varying formation rates.
