This thesis deals with a very specific class of molecular sieves known as zeolites. Zeolites are a class of crystalline aluminosilicates characterised by pores or cavities of molecular dimensions as part of their crystal structure. In this work zeolites were modified for the use and understanding of different catalytic applications. Primarily the zeolites were modified regarding the porosity and the introduction of metals to the framework. The obtained materials were used as solid acid catalysts, as an inert matrix for stabilising metal nanoparticles and as an anchoring material for molecular metal oxide species. Nanosized and mesoporous zeolites were prepared to investigate the effect of inter- or intracrystalline mesopores on the catalytic lifetime in the conversion of methanol to hydrocarbons (MTH). It was found that the mesoporous zeolite with intracrystalline mesopores displayed the significantly longest catalytic lifetime compared to the nanosized zeolites and the conventional counterpart. Even though the introduction of mesopores improved the catalytic lifetime in the MTH reaction it was concluded that the normal benefits from desilication, e.g. mesoporosity and repairing of defects, became masked by the generation of extra-framework aluminum and that the catalytic lifetime was severely dependent on the amount of extra-framework aluminum. Conventional and mesoporous ZSM-5 zeolites were prepared together with the Ga-MFI zeotype analogues to investigate the differences in activity, selectivity and mode of deactivation. The differences in selectivity were primarily ascribed to the difference in the lower acidity of the individual active sites of the Ga-MFI zeotypes compared to the zeolites. In general, the Ga-MFI zeotypes deactivated faster than the ZSM-5 zeolites. Further investigations of the mode of deactivation revealed that the zeolites deactivated due to coke formation and that the Ga-MFI zeotypes deactivated due to loss of the catalytically active Brønsted acid sites caused by hydrolysis of Ga-O bonds leading to formation of inactive extra-framework gallium. Zeolites can not only be used as solid acid catalysts but can also be used as a size-selective matrix. It was shown that it is possible to encapsulate 1-2 nm sized gold nanoparticles by silicalite-1 or ZSM-5 zeolite crystals thereby forming a sintering-stable and substrate size-selective oxidation catalyst. After carrying out calcination experiments, both in situ and ex situ indicated that the gold nanoparticles embedded in the crystals were highly stable towards sintering. The catalytic tests proved that the embedded gold nanoparticles were active in selective aldehyde oxidation and were only accessible through the micropores. Furthermore, preliminary work was done using mesoporous ZSM-5 zeolites as support material for anchoring molecular CoMo6 species for the application as potential bi-functional catalyst in simultaneous hydrodesulfurisation (HDS) and hydrocracking. HDS activity tests revealed that the anchoring improved the activity compared to an impregnated counterpart.