1. Introduction.- 2. Techniques.- 3. Atomic Structure and Spectra.- 4. Transition Probabilities.- 5. Radiative Processes.- 6. Collisional Processes.- Appendices.- A.1 Numerical Data for Electronic Binding Energies.- A.2 Numerical Data for Electromagnetic Decay.- A.3 Spectroscopic Data for X-Ray Lasers.- References.

The physics of highly charged ions continues to be one of the most active and interesting fields of atomic physics. A large fraction of the characteristic radiation of such ions lies in the x-ray region and its spectroscopy represents an important experimental tool. The field of x-ray spectroscopy grew directly from the discovery of x radiation by Wilhelm Conrad Rontgen in 1895. The early contributions to atomic physics that arose out of x-ray spectroscopy are well documented and are the subject of many centennial events. In the past, the gross features of most x-ray spectra in the hard x-ray region have been accounted for on a hydrogenic model. In many instances the gross spectral features recorded in the early days of x-ray physics match those observed with state-of-the-art techniques today and many of the early qualitative - terpretations have remained unchanged. It is in the details of the spectra that today's results are superior to those obtained many years ago, and it is in the quantitative and accurate - scriptions that today's predictions are better. A rejuvenation of the field has occurred after the great achievements in the development of new ion sources for production of heavy ions with only one or few electrons. The new tools available to the experimenter allow the exploration of new states of m- ter and allow us to challenge new frontiers in our theoretical understanding of atoms and their interactions with other particles.