  Isospin symmetry is explicitly broken in the Standard Model by the non-zero
differences of mass and electric charge between the up and down quarks. Both of
these corrections are expected to have a comparable size of the order of one
percent relatively to hadronic energies. Although these contributions are
small, they play a crucial role in hadronic and nuclear physics. In this review
we explain how to properly define QCD and QED on a finite and discrete
space-time so that isospin corrections to hadronic observables can be computed
ab-initio. We then consider the different approaches to compute lattice
correlation functions of QCD and QED observables. Finally we summarise the
actual lattice results concerning the isospin corrections to the light hadron
spectrum.
