Concerning lattice approaches to quark flavour physics, there are
quantities that we know how to calculate and that are currently well
known. Improving the overall accuracy on these observables (decay
constants, semileptonic form factors, bag parameters, etc.) requires
the inclusion of effects that have been neglected up to know. Among
these there are the isospin breaking corrections due to the difference
between the up and down quark masses (particularly relevant in the
case of Kl3 decays). I shall discuss a method to calculate isospin
breaking corrections and show some preliminary results on hadron
spectrum, decay constants and semileptonic form factors.