  Quarks of other flavours than up and down, i.e. $s$, $c$, and $b$ quarks,
have been long recognized as effective probes of the structure of hot QCD
matter. In this talk, I review some of the motivations for their investigation
and discuss the salient results obtained so far, with a focus on the results
from the Relativistic Heavy Ion Collider (RHIC). Some ideas for future studies
are also mentioned.
