  High-transition temperature (high-$T_c$) superconductivity in the iron
pnictides/chalcogenides emerges from the suppression of the static
antiferromagnetic order in their parent compounds, similar to copper oxides
superconductors. This raises a fundamental question concerning the role of
magnetism in the superconductivity of these materials. Neutron scattering, a
powerful probe to study the magnetic order and spin dynamics, plays an
essential role in determining the relationship between magnetism and
superconductivity in high-$T_c$ superconductors. The rapid development of
modern neutron time-of-flight spectrometers allows a direct determination of
the spin dynamical properties of iron-based superconductors throughout the
entire Brillouin zone. In this review, we present an overview of the neutron
scattering results on iron-based superconductors, focusing on the evolution of
spin excitation spectra as a function of electron/hole-doping and isoelectronic
substitution. We compare spin dynamical properties of iron-based
superconductors with those of copper oxide and heavy fermion superconductors,
and discuss the common features of spin excitations in these three families of
unconventional superconductors and their relationship with superconductivity.
