Lifetimes and oscillations of heavy mesons

We review the theoretical status of the lifetime ratios $\tau_{B^+} / \tau_{B_d}$ and $\tau_{B_s} / \tau_{B_d}$ and of the mixing quantities $\Delta M_s$, $\Delta \Gamma_s$ and $\phi_s$. We show that the ratio $\Delta \Gamma_s / \Delta M_s$ can be determined with almost no non-perturbative uncertainties. Finally we explain how this precise determination of the standard model values can be used to find possible new physics contributions in $\Delta M_s$, $\Delta \Gamma_s$, $\Delta \Gamma_s / \Delta M_s$ and $a_{fs}^s$. Combining the latest experimental bounds on these quantities one already gets some hints for new physics contributions.