A century of X rays in atomic physics

Photon-atom interactions have played an essential role in leading to present understanding of atomic structure and dynamics. The discovery of x rays, of characteristic radiation and of Auger transitions opened inner shells to exploration and greatly advanced the field. More recently, synchrotron-radiation sources and increasingly sophisticated instrumentation are bringing about a remarkable enhancement of experimental capability. Tempting opportunities arise for new investigations of many-body effects, fundamental processes, and relativistic and quantum electrodynamic phenomena. Some aspects of the history and present status of the subject are outlined.