  We present a QCD-based approach to the endpoint region of the lepton spectrum
in $\bar B\to X_u\,\ell\,\bar\nu$ decays. We introduce a genuinely
nonperturbative form factor, the shape function, which describes the fall-off
of the spectrum close to the endpoint. The moments of this function are related
to forward scattering matrix elements of local, higher-dimension operators. We
find that nonperturbative effects are dominant over a finite region in the
lepton energy spectrum, the width of which is related to the kinetic energy of
the $b$-quark inside the $B$ meson. Applications of our method to the
extraction of fundamental standard model parameters, among them $V_{ub}$, are
discussed in detail.
