Heavy Flavour Decays - Introduction and Overview

We review some selected topics in the decays of heavy flavours, beauty and charm, which are of principal interest at HERA-B and HERA. The topics in $B$ physics include: an update on the quark mixing matrix and CP violating phases, issues bearing on an improved resolution on the CP-violating phase $\Delta (sin 2 \beta)$, prospects of measuring radiative and semileptonic rare $B$ decays, the $B_s^{0}$ - $\overline{B_s^{0}}$ mixing ratio $x_s$, improved measurements of the $B_d^{0}$ - $\overline{B_d^{0}}$ mixing ratio $x_d$ and the $B$-hadron lifetimes, in particular $\tau(\Lambda_b)$. In the charm sector, we have focussed on rare decays and $D^0$- $\overline{D^0}$ mixing, whose measurements will signal physics beyond the standard model.