Up to the 1980s, the phenomena occurring in the context of the interaction of atoms and radiation were theoretically well described by perturbation theory. Within the past decade, however, laser sources with peak intensities of the order of 10 ¹⁶ W/cm² have become experimentally feasible. In this intensity regime, the external laser field is comparable to the binding energies of the electrons, and therefore it can no longer be treated as a perturbation. The inadequacy of this theory has also been confirmed by several experimental observations concerning high-intensity optical phenomena, already for fields of the order of 10 ¹³ W/cm². Therefore, matter in strong laser fields poses now a great challenge to both theoretical and experimental physicists, such that this field of research constitutes one of the most active areas within atomic physics.

Apart from the understanding of the main phenomena in this intensity regime, such as high-order harmonic generation and ionization, applications are for instance plasma physics (in particular fusion), particle physics, and x-ray sources. Furthermore, since the physical mechanisms behind strong-field phenomena take place within hundreds of attoseconds (10^-18s), they constitute powerful tools for resolving or even controlling dynamic processes with subfemtosecond precision. Concrete examples are dynamic imaging of, for instance, molecular systems, with attosecond precision, or  the generation of attosecond pulses.