The Lifetimes of Heavy Flavour Hadrons - a Case Study in Quark-Hadron Duality

The status of heavy quark expansions for charm and beauty lifetime ratios is reviewed. Taking note of the surprising semiquantitative success of this description for charm hadrons I interprete the new data on $\tau (D_s)$ and re-iterate the call for more precise measurements of $\tau ({\mathrm{\Xi }}_c^{0,+})$ and $\tau ({\mathrm{\Omega }}_c)$. A slightly larger $B^{-}$ than $B_d$ lifetime is starting to emerge as predicted; the largest lifetime difference in the beauty sector, namely in $\tau (B_c)$ vs. $\tau (B)$ has correctly been predicted; the problem posed by the short ${\mathrm{\Lambda }}_b$ lifetime remains. The need for more accurate data also on $\tau (B_s)$ and $\tau ({\mathrm{\Xi }}_b^{-,0})$ is emphasized. I discuss quark-hadron duality as the central theoretical issue at stake here.