  Strong electronic correlations are often associated with the proximity of a
Mott insulating state. In recent years however, it has become increasingly
clear that the Hund's rule coupling (intra-atomic exchange) is responsible for
strong correlations in multi-orbital metallic materials which are not close to
a Mott insulator. Hund's coupling has two effects: it influences the energetics
of the Mott gap and strongly suppresses the coherence scale for the formation
of a Fermi-liquid. A global picture has emerged recently, which emphasizes the
importance of the average occupancy of the shell as a control parameter. The
most dramatic effects occur away from half-filling or single occupancy. The
theoretical understanding and physical properties of these `Hund's metals' are
reviewed, together with the relevance of this concept to transition-metal
oxides of the 3d, and especially 4d series (such as ruthenates), as well as to
the iron-based superconductors (iron pnictides and chalcogenides).
