  The surprising discovery of superconductivity in layered iron-based
materials, with transition temperatures climbing as high as 55 K, has lead to
thousands of publications on this subject over the past two years. While there
is general consensus on the unconventional nature of the Cooper pairing state
of these systems, several central questions remain - including the role of
magnetism, the nature of chemical and structural tuning, and the resultant
pairing symmetry - and the search for universal properties and principles
continues. Here we review the progress of research on iron-based
superconducting materials, highlighting the major experimental benchmarks that
have been so far reached and the important questions that remain to be
conclusively answered.
