  The $f^+$ form factors of the $B\to \pi$, $D\to \pi$ and $D\to K$ transitions
are calculated from QCD light-cone sum rules (LCSR) and used to predict the
widths and differential distributions of the exclusive semileptonic decays
$B\to \pi \bar{l}\nu_l$, $D \to\pi \bar{l}\nu_l$ and $D \to K \bar{l}\nu_l$,
where $l=e,\mu$. The current theoretical uncertainties are estimated. The LCSR
results are found to agree with the results of lattice QCD calculations and
with experimental data on exclusive semileptonic D decays. Comparison of the
LCSR prediction on $B\to \pi \bar{l} \nu_l$ with the CLEO measurement yields a
value of |V_{ub}| in agreement with other determinations.
