Compact XUV and X Ray sources from laser plasma interactions : theoretical and numerical study In this thesis the generation of high order harmonics of ultrashort and high intensity laser pulses from solid density plasmas so called surface high harmonic generation (SHHG) is studied. With SHHG a compact source of coherent XUV and X Ray radiation becomes possible. The results are obtained numerically using 1D and 2D Particle In Cell (PIC) computer simulations which are supported by analytical models. This work focusses on two main issues of SHHG to date pulse isolation and generation efficiency. It is shown that a single attosecond pulse (AP) can be obtained from a few cycle incident laser pulse by choosing a suitable carrier envelope phase (CEP) depending on the density and shape of density gradient of the target. An analytical model providing an interpretation of the results obtained from PIC simulations is presented. Spatial isolation of APs can be achieved using the attosecond lighthouse effect but surface denting is detrimental to the separation of APs. PIC simulations are used to explain an experimental result where a separation of pulses was not possible due to surface denting. Furthermore it is shown that the angular spectral chirp corresponds to the depth of the surface denting. The efficiency of SHHG can be enhanced greatly by reflecting the beam coming from a first target off a second target. Of major importance for the efficiency is the relative phase between harmonics on the surface of the second target. The relative phase changes even when propagating in free space due to the Gouy phase. To maximize the efficiency gain a parametric study using PIC simulations has been performed to find the optimal distance between two targets.