Three recent experiments on multiphoton ionisation of atoms are interpreted. They show both resonant and non-resonant behaviour, and fit the predictions of a general theory. It is suggested that clean and unambiguous measurements of atomic parameters in strong laser fields can be obtained using extensions of these experiments.
Four-photon ionisation of caesium requires strong electromagnetic fields and the usual interpretation in terms of generalised cross sections has to be questioned. A theoretical calculation in the experimental conditions of a recent measurement provides a new interpretation of the results of Morellec and Normand (1979): the effect of light shifts masks the effect of saturation and leads one to overestimate the multiphoton ionisation cross section. The expected overestimation is only 25 per cent for the quoted experiment, but could be larger in some other physical cases.