  Hadrons carrying heavy quarks, i.e. charm or bottom, are important probes of
the hot and dense medium created in relativistic heavy ion collisions. Heavy
quark-antiquark pairs are mainly produced in initial hard scattering processes
of partons. While some of the produced pairs form bound quarkonia, the vast
majority hadronize into particles carrying open heavy flavor. Heavy quark
production has been studied by the PHENIX experiment at RHIC via measurements
of single leptons from semi-leptonic decays in both the electron channel at
mid-rapidity and in the muon channel at forward rapidity. A large suppression
and azimuthal anisotropy of single electrons have been observed in Au+Au
collisions at 200 GeV. These results suggest a large energy loss and flow of
heavy quarks in the hot, dense matter. The PHENIX experiment has also measured
${J/\psi}$ production at $\sqrt{s_{nn}}$ = 200 GeV in p+p, d+Au, Cu+Cu and
Au+Au collisions, both at mid- and forward-rapidities, and additionally Cu+Au
and U+U at forward-rapidities. In the most energetic collisions, more
suppression is observed at forward rapidity than at central rapidity. This can
be interpreted either as a sign of quark recombination, or as a hint of
additional cold nuclear matter effects. The centrality dependence of nuclear
modification factor, RAA, for ${J/\psi}$ in U+U collisions at $\sqrt{s_{nn}}$ =
193 GeV shows a similar trend to the lighter systems, Au+Au and Cu+Cu, at
similar energy 200 GeV.
