The primordially dominated production of heavy quarks
renders them valuable probes of the subsequently formed
medium in high-energy collisions of heavy nuclei. Charm
and bottom quarks are thus well suited to test energy-loss and thermalization mechanisms over the entire pt-range, to
disentangle radiative from elastic scattering processes,
and to study both transport and microscopic properties of
the (s)QGP. A survey of theoretical and phenomenological aspects of recent approaches is given, and implications
for a consistent understanding of heavy-quarkonium observables are identified.