Optical physics is concerned with the dynamical interactions of atoms and molecules with electromagnetic to fields. Semiclassical theories, which study the interaction of atoms with classical fields, are often said to comprise optical physics, while quantum optics treats the interaction of atoms or molecules with quantized electromagnetic fields. A significant part of optical physics and quantum optics is the study of near-resonant atom-field interactions, and concentrates on nonperturbative dynamics, where the effects of the optical fields have to be kept to all orders. The atomic properties themselves are assumed to be known. The vast majority of problems in light-matter interactions can be treated quite accurately within light-matter interactionsemiclassical approximationlight-matter interactionsemiclassicalquantumopticslight-matter interactionquantized fields semiclassical theories. However, an important class of problems where this is not the case are presented in Chapt. 78. While much of optical physics and quantum optics ignores the effects of the electromagnetic fields on the center-of-mass motion of the atoms, important topics such as atomic trapping and cooling (Chapt. 75) and de Broglie optics (Chapt. 77) rely in an essential way on such mechanical effects of light. The present chapter deals with more “traditional” aspects of optical physics, where these effects are ignored.