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Relativistic and quantum mechanical effects in the interaction of atomic systems with a superintense laser field are investigated. Various atomic systems are the object of attention, ranging from highly charged ions to thin crystal layers. A new regime of high-harmonic generation (HHG) based on multiply charged ions is described, where the dynamics of the tunneled wave packet of the electron between birth and recombination is strongly modified by the ionic core, producing a coherent burst of hard X-rays. We also consider relativistic effects in the nonsequential double ionization of helium. The scheme for phase-matched HHG from a laser-driven thin crystal layer in the presence of a high background of free electrons is discussed. For a selective range of parameters, the initial regular structure of the layer is preserved during the strong interaction with the laser pulse due to the suppression of shock waves in the layer. As a result, short hard x-ray pulses may be generated with unprecedented coherence, and thus the possibility of applications in x-ray spectroscopy and holography. [ABSTRACT FROM AUTHOR]