Chiral Perturbation Theory with Photons and Leptons

I discuss a low-energy effective field theory which permits the full treatment of isospin-breaking effects in semileptonic weak interactions. In addition to the pseudoscalars and the photon, also the light leptons have to be included as dynamical degrees of freedom in an appropriate chiral Lagrangian. I describe the construction of the local action at next-to-leading order.