Uncertainties in the Determination of $\mathrm{\mid }{V}_{cb}\mathrm{\mid }$

I discuss the theoretical uncertainties in the extraction of $\mathrm{\mid }{V}_{cb}\mathrm{\mid }$ from semilep-to-nic decays of $B$ mesons, taking into account the most recent theoretical developments. The main sources of uncertainty are identified both for the exclusive decay mode $B\to D^{\ast }\mathrm{\ell }\bar{\nu }$ and for the inclusive channel $B\to X\mathrm{\ell }\bar{\nu }$. From an analysis of the available experimental data, I obtain $|,V_{cb}|{\rm excl} = 0.041\pm 0.003{\rm exp}\pm 0.002_{\rm th}$fromtheexclusivemode,and$|,V_{cb}|{\rm incl} = 0.040\pm 0.001{\rm exp}\pm 0.005_{\rm th}$ from the inclusive mode. I also give a prediction for the slope of the form factor $\mathcal{F}(w)$ at zero recoil, which is ${\widehat{\varrho }}^2=0.8\pm 0.3$.