To reach a high resolution for X-ray computed tomography (XCT), the whole system should be as stable as possible. To reduce focal spot and system movements, a good knowledge of possible influences in a certain terrain should be gained. Some of these possible influences and how to reduce them are presented in this work. The main part of this work deals with high resolution scans of composites and how to quantify them. For glass fibre reinforced polymers (GFRP) a voxel size <2 µm is necessary, to extract a correct fibre length distribution. To evaluate the process chain from a wood polymer composite over a C-template to the final C/Si/SiC-ceramic by means of XCT voxel sizes down to 0.8 µm are necessary. For thermo-mechanically treated Mg-alloys, voxel sizes down to 0.66 µm were used to obtain the 3D-structure of 4-6 different phases in one alloy. To verify XCT-Results, reference methods (e.g.: pyrolysis, synchrotron XCT, optical microscopy or scanning electron microscopy) were applied.