We are performing basic research in a very exciting field at the border between atomic and molecular physics and advanced optics, nonlinear optics and laser physics: high-order harmonic generation in gaseous media exposed to intense laser fields and its applications. The harmonic spectrum exhibits an extended "plateau" where consecutive (odd) harmonics have approximately the same intensity. If the harmonics are emitted in phase, i.e. phase-locked, the temporal structure of the radiation emitted from the medium consists of a “train” of attosecond pulses separated by half the laser period.