Isospin symmetry is not exact and the corrections to the isosymmetric limit
are, in general, at the percent level. For gold plated quantities, such as
pseudoscalar meson masses or the kaon leptonic and semileptonic decay rates,
these effects are of the same order of magnitude of the errors quoted in
nowadays lattice calculations and cannot be neglected any longer. In this talk
I discuss the methods that have been developed in the last few years to
calculate isospin breaking corrections by starting from first principles
lattice simulations. In particular, I discuss how to perform a combined QCD+QED
lattice simulation and a renormalization prescription to be used in order to
separate QCD from QED isospin breaking effects. A brief review of recent
lattice results of isospin breaking effects on the hadron spectrum is also
included.Comment: 15 pages, plenary talk presented at the 31st International Symposium
on Lattice Field Theory (Lattice 2013), 29 July - 3 August 2013, Mainz,
German