My intention in writing this book was to revisit some theoretical methods related to the interaction of electro-magnetic fields with elementary quantum systems. From a broader viewpoint， this scientific area is perhaps the oldest one （in fact， the blackbody radiation problem boosted the birth of quantum mechanics as a physical theory of atomic-sized systems） and as such it is a very mature field. So， in principle， there are not many topics left to talk about without actually revisiting them. After ninety years of quantum mechanics and about sixty years from the firing-up of the first laser， most theoretical methods of relevance are probably already known. Now and then some of them re-appear under different names， but a closer look reveals that it is not in fact a question of a 'first-seen' approach. Thus， 'why take on the hassle and write another book on a not-so-modern subject （and consequently not an interesting topic among the leaders of technology and fund hunters）? One answer is that I have the feeling that it is precisely this wealth of， seemingly different， methods and approaches that constitutes a serious obstacle for newcomers to this field （and others）. The connection of the （normally） heavily formalized methods with the simpler physical principles is fuzzy and almost never mentioned in the specialized literature， as it is taken for granted， or， when it is referenced in the form of a citation， it soon leads to a stochastic branching process problem in tracing the labyrinthine literature. As a result， although quick access to the scientific literature is now guaranteed， in my view this is often not helpful in clarifying matters and eventually the underlying physical context is left in the distant past. This why the phraseology of the research literature sometimes looks like the reinvention of the wheel.