  We discuss the theoretical framework required for the computation of
radiative corrections to semileptonic decay rates in lattice simulations, and
in particular to those for $K_{\ell3}$ decays. This is an extension of the
framework we have developed and successfully implemented for leptonic decays.
New issues which arise for semileptonic decays, include the presence of
unphysical terms which grow exponentially with the time separation between the
insertion of the weak Hamiltonian and the sink for the final-state meson-lepton
pair. Such terms must be identified and subtracted. We discuss the cancellation
of infrared divergences and show that, with the QED$_\mathrm{\,L}$ treatment of
the zero mode in the photon propagator, the $O(1/L)$ finite-volume corrections
are "universal". These corrections however, depend not only on the semileptonic
form factors $f^\pm(q^2)$ but also on their derivatives $df^\pm/dq^2$. (Here
$q$ is the momentum transfer between the initial and final state mesons.) We
explain the perturbative calculation which would need to be performed to
subtract the $O(1/L)$ finite-volume effects.
