The preceding chapters have been concerned with the properties of the radiation field alone. In this chapter we turn to the interaction between radiation and matter. This is of course the domain of quantum electrodynamics, however in quantum optics we are usually only concerned with low energy systems of bound electrons which simplifies matters considerably. We will use the occupation number representation for bound many-electron systems to quantize the electronic degrees of freedom, following the approach of Haken (Waves, Photons and Atoms. North Holland, Amsterdam, vols. 1 and 2, 1981, [1]) and also Cohen-Tannoudji (Photons and Atoms: Introduction to Quantum Electrodynamics. Wiley-Interscience, 1997, [2]).