  When aiming at the percent precision in hadronic quantities calculated by
means of lattice simulations, isospin breaking effects become relevant. These
are of two kinds: up/down mass splitting and electromagnetic corrections. In
order to account properly for the latter, a consistent formulation of
electrically-charged states in finite volume is needed. In fact on a periodic
torus Gauss law and large gauge transformations forbid the propagation of
electrically-charged states. In this talk I will review methods that have been
used or proposed so far in order to circumvent this problem, while highlighting
practical as well as conceptual pros and cons. I will also review and discuss
various methods to calculate electromagnetic corrections to hadron masses and
decay rates in numerical simulations.
