  The peculiar role of heavy-flavour observables in relativistic heavy-ion
collisions is discussed. Produced in the early stage, $c$ and $b$ quarks cross
the hot medium arising from the collision, interacting strongly with the
latter, until they hadronize. Depending on the strength of the interaction
heavy quarks may or not approach kinetic equilibrium with the plasma, tending
in the first case to follow the collective flow of the expanding fireball. The
presence of a hot deconfined medium may also affect heavy-quark hadronization,
being possible for them to recombine with the surrounding light thermal
partons, so that the final heavy-flavour hadrons inherit part of the flow of
the medium. Here we show how it is possible to develop a complete transport
setup allowing one to describe heavy-flavour production in high-energy nuclear
collisions, displaying some major results one can obtain. Information coming
from recent lattice-QCD simulations concerning both the heavy-flavour transport
coefficients in the hot QCD plasma and the nature of the charmed degrees around
the deconfinement transition is also presented. Finally, the possibility that
the formation of a hot deconfined medium even in small systems
(high-multiplicity p-Au and d-Au collisions, so far) may affect also
heavy-flavour observables is investigated.
