Modern methods of laser spectroscopy allow the study of single atoms or ions in an unperturbed environment. This has opened up interesting new experiments, an among them is the detailed study of radiation-atom coupling. This chapter reviews the following two experiments of this type: the single-atom maser and the study of the resonance fluorescence of a single stored ion. The simplest and most fundamental system for studying radiation-matter coupling is a single two-level atom interacting with a single mode of an electromagnetic field in a cavity. There is another aspect of the single-atom maser which is very interesting: the non-classical statistics of the photons in the cavity. The results obtained for the single-atom maser also give a new insight into the statistical properties of masers and lasers. A problem in radiation-matter interaction which has received a lot of interest is resonance fluorescence. The experiments to investigate antibunching in resonance fluorescence have been performed by means of laser-excited collimated atomic beams.