This volume presents a collection of papers, written during the last 30 years by Claude Cohen-Tannoudji and his collaborators, on various physical effects which can be observed on atoms interacting with electromagnetic fields. It consists of a personal selection of review papers, lectures given at schools, as well as original experimental papers. Emphasis is put on physical mechanisms and on general approaches, such as the dressed atom approach, having a wide range of applications. Various topics are discussed, such as light shifts, level crossing resonances, multiphoton processes, resonance...