Mixing and lifetimes of $b$-hadrons

We review the status of mixing and lifetimes of $b$-hadrons. We will show that $\Delta \Gamma / \Delta M$, $a_{sl}$ and $\phi$ are better suited to search for new physics effects than $\Delta M$ alone, because of our poor knowledge of the decay constants. The theoretical precision in the determination of $\Gamma_{12} / M_{12}$ - which contains all information on $\Delta \Gamma / \Delta M$, $a_{sl}$ and $\phi$ - can be tested directly by investigating the lifetimes of $b$-hadrons, because both quantities rely on the same theoretical footing. In particular we will also present a numerical estimate for the lifetime of the $\Xi_b$-baryon.