Isospin symmetry is an almost exact property of the Standard Model. This happens because the difference between the up and down quark masses is much smaller than a typical hadronic scale and because QED radiative corrections are suppressed by the smallness of the electromagnetic coupling constant. For these reasons, most of the existing lattice QCD predictions for several physical quantities have been obtained by assuming exact isospin symmetry. With the increasing precision of both the experimental and theoretical determinations of quantities such as the Kl2 and Kl3 decay rates, a good control over isospin breaking effects is becoming phenomenologically relevant. In this talk I shall discuss a method to calculate on the lattice isospin breaking effects and, more generally, QED radiative corrections to hadronic matrix elements. I shall show numerical results for the charged-neutral pion and kaon mass splittings and discuss how the QED radiative corrections to the leptonic decay rate of a charged pseudoscalar meson can be calculated on the lattice by starting from euclidean correlators.