  The new family of unconventional iron-based superconductors discovered in
2006 immediately relieved their copper-based high-temperature predecessors as
the most actively studied superconducting compounds in the world. The
experimental and theoretical effort made in order to unravel the mechanism of
superconductivity in these materials has been overwhelming. Although our
understanding of their microscopic properties has been improving steadily, the
pairing mechanism giving rise to superconducting transition temperatures up to
55 K remains elusive. And yet the hope is strong that these materials, which
possess a drastically different electronic structure but similarly high
transition temperatures compared to the copper-based compounds, will shed
essential new light onto the several-decade-old problem of unconventional
superconductivity. In this work we review the current understanding of the
itinerant-charge-carrier dynamics in the iron-based superconductors and parent
compounds largely based on the optical-conductivity data the community has
gleaned over the past seven years using such experimental techniques as
reflectivity, ellipsometry, and terahertz transmission measurements and analyze
the implications of these studies for the microscopic properties of the
iron-based materials as well as the mechanism of superconductivity therein.
