  Superconductivity develops in metals upon the formation of a coherent
macroscopic quantum state of electron pairs. Iron pnictides and chalcogenides
are materials that have high superconducting transition temperatures. In this
Review, we describe the advances in the field that have led to higher
superconducting transition temperatures in iron-based superconductors and the
wide range of materials that form them. We summarize both the essential aspects
of the normal state and the mechanism for superconductivity. We emphasize the
degree of electron-electron correlations and their manifestation in properties
of the normal state. We examine the nature of magnetism, analyse its role in
driving the electronic nematicity, and discuss quantum criticality at the
border of magnetism in the phase diagram. Finally, we review the amplitude and
structure of the superconducting pairing, and survey the potential settings for
optimizing superconductivity.
