The response of many-particle systems to intense lasers has been of topical interest since the advent of femtosecond laser pulses. In particular, atomic clusters are extensively studied experimentally and theoretically.

I will discuss theoretical approaches to the dynamics of rare-gas clusters and compare investigations in the infrared regime to those made for X-ray impact. Whereas the infrared (long-wavelength) regime shows an interesting electron dynamics on a sub-femtosecond time scale, the X-ray (short-wavelength) regime is of interest in view of future imaging applications.