  The precision of lattice QCD computations of many quantities has reached such
a precision that isospin-breaking corrections, including electromagnetism, must
be included if further progress is to be made in extracting fundamental
information, such as the values of Cabibbo-Kobayashi-Maskawa matrix elements,
from experimental measurements. We discuss the framework for including
radiative corrections in leptonic and semileptonic decays of hadrons, including
the treatment of infrared divergences. We briefly review isospin breaking in
leptonic decays and present the first numerical results for the ratio
$\Gamma(K_{\mu2})/\Gamma(\pi_{\mu2})$ in which these corrections have been
included. We also discuss the additional theoretical issues which arise when
including electromagnetic corrections to semileptonic decays, such as
$K_{\ell3}$ decays. The separate definition of strong isospin-breaking effects
and those due to electromagnetism requires a convention. We define and advocate
conventions based on hadronic schemes, in which a chosen set of hadronic
quantities, hadronic masses for example, are set equal in QCD and in QCD+QED.
This is in contrast with schemes which have been largely used to date, in which
the renormalised $\alpha_s(\mu)$ and quark masses are set equal in QCD and in
QCD+QED in some renormalisation scheme and at some scale $\mu$.
