  High-temperature superconductivity in both the copper-oxide and the
iron-pnictide/chalcogenide systems occurs in close proximity to
antiferromagnetically ordered states. Neutron scattering has been an essential
technique for characterizing the spin correlations in the antiferromagnetic
phases and for demonstrating how the spin fluctuations persist in the
superconductors. While the nature of the spin correlations in the
superconductors remains controversial, the neutron scattering measurements of
magnetic excitations over broad ranges of energy and momentum transfer provide
important constraints on the theoretical options. We present an overview of the
neutron scattering work on high-temperature superconductors and discuss some of
the outstanding issues.
