Description
The Belle and Belle II experiments have collected a combined sample of 1.2~ab$^{-1}$ of $e^+ e^-\to B\bar{B}$ collisions at a centre-of-mass energy corresponding to the $\Upsilon(4S)$ resonance. These data, with low particle multiplicity and constrained initial state kinematics, are an ideal environment for studying semileptonic and leptonic decays of the $B$ meson. Combined with theoretical inputs, measurements of both inclusive and exclusive semileptonic decays yield information about the Cabibbo-Kobayashi-Maskawa matrix elements $V_{cb}$ and $V_{ub}$. Our latest results based on the Belle II data set are reviewed. We also present the first measurement of $B^{+}\to \tau^{+}\nu$ from Belle II and a search for $B^+\to \mu^+\nu$ using the combined Belle and Belle~II samples. These decays provide constraints on beyond-the-standard model physics and provide alternative measurements of $V_{ub}$ that complement those from semileptonic decay.
