The recent development of lasers capable of delivering short pulses of very intense radiation, over a frequency range extending from the infrared to the ultraviolet, has led to the discovery of new phenomena in laser interactions with atomic systems. In the first part of this article, a survey is given of the basic properties found in the study of multiphoton processes such as the multiphoton ionization of atoms, the emission by atoms of high order harmonics of the exciting laser light and laser-assisted electron-atom collisions. The second part contains a review of the main non-perturbative methods, based on the Floquet approach or the numerical solution of the time-dependent equations for the wave function, which have been used to perform theoretical studies of these processes. ©1998 American Institute of Physics.