High-order harmonic generation in strong laser fields is considered one of the most promising ways of producing coherent and intense X-rays. In this project, we want to explore theoretically two aspects of harmonic generation. The first part is the study of harmonics generated by two-color lasers, and the exploration of the possibility to control both the single-atom dynamics and the properties of the emitted fields. An exciting idea, which we will explore, is the production of subfemtosecond pulses of light by combining two laser pulses of close frequencies and crossed polarizations. The second part of the project includes the study of nonlinear response of complex atoms to strong XUV harmonic irradiation, in parallel to the experiments. We want to investigate, for example, whether harmonics of this XUV light can be produced with a reasonable probability. This could be a way towards the production of coherent X-ray radiation beyond 150 eV.