Multiphoton processes in atoms in intense laser-light fields is gaining ground as a spectroscopic diagnostic tool. This text presents descriptions of processes occurring in atoms under the action of strong electromagnetic radiation, in particular, the shift, broadening and mixing of atomic states. The topics covered include tunnelling ionization, above-threshold ionization, ionization of multiply charged ions, resonance-enhanced ionization, super-intense radiation fields, and properties of Rydberg states strongly perturbed by laser radiation.