I will discuss a method to evaluate on the lattice leading isospin breaking effects due to both the small mass difference between the up and down quarks and to the QED interaction. The method is applicable in principle to any QCD+QED gauge invariant hadronic observable which can be computed on the lattice. It is based on the expansion of the path-integral in powers of the small parameters md-mu and alpha_em. I will show preliminary results for the pions mass splitting, for the kaons mass splitting and for the Dashen's theorem breaking parameter epsilon_gamma. I will also show updated results for the QCD isospin breaking effects on the Kl2 decay rate and for the QCD contribution to the neutron--proton mass difference.