  Superconductivity is a remarkably widespread phenomenon observed in most
metals cooled down to very low temperatures. The ubiquity of such conventional
superconductors, and the wide range of associated critical temperatures, is
readily understood in terms of the celebrated Bardeen-Cooper-Schrieffer (BCS)
theory. Occasionally, however, unconventional superconductors are found, such
as the iron-based materials, which extend and defy this understanding in new
and unexpected ways. In the case of the iron-based superconductors, this
includes a new appreciation of the ways in which the presence of multiple
atomic orbitals can manifest in unconventional superconductivity, giving rise
to a rich landscape of gap structures that share the same dominant pairing
mechanism. Besides superconductivity, these materials have also led to new
insights into the unusual metallic state governed by the Hund's interaction,
the control and mechanisms of electronic nematicity, the impact of magnetic
fluctuations and quantum criticality, and the significance of topology in
correlated states. Over the thirteen years since their discovery, they have
proven to be an incredibly fruitful testing ground for the development of new
experimental tools and theoretical approaches, both of which have extensively
influenced the wider field of quantum materials.
