  The diffusion of heavy quarks through the quark-gluon plasma (QGP) as
produced in high-energy heavy-ion collisions has long been recognized as an
excellent probe of its transport properties. In addition, the experimentally
observed heavy-flavor hadrons carry valuable information about the
hadronization process of the transported quarks. Here we review recent progress
in the theoretical developments of heavy-quark interactions in the QGP and how
they relate to the nonperturbative hadronization process, and discuss the
recent status of the pertinent phenomenology in heavy-ion collisions at the
RHIC and the LHC. The interactions of heavy quarks in the QGP also constitute a
central building block in the description of the heavy quarkonia which controls
their transport parameters as well. We will thus focus on theoretical
approaches that aim for a unified description of open and hidden heavy-flavor
particles in medium, and discuss how they can be constrained by lattice QCD
"data" and utilized to deduce fundamental properties of the microscopic
interactions and emerging spectral properties of the strongly coupled QGP.
