Today, adhesively bonded joints are often favored over mechanical joining in high-end applications, due to their better stress distribution, lower weight and the ability to make dissimilar joints. However, neat adhesives suffer from a lower mechanical strength compared to mechanical fasteners. To remedy this, nanoparticles have been successfully added as toughening agents. Multiple, often competing, toughening mechanisms have been proposed for this, in which the particle size and the interaction with the polymer matrix play an important role. Yet, few studies focused on the influence of these parameters. This project aims at elucidating the effects of particle size and surface chemistry on the toughening behavior in nanoreinforced adhesives. To this end, hafnium oxide (HfO2) nanocrystals will be synthesized in different size ranges and subsequently modified with a library of ligands. These nanocrystals will then be dispersed in different industrially important adhesive types, being epoxy, polyurethane and acrylics. Subsequent assessment of the mechanical properties in composite and composite/metal multi-material joints and visualization allows then to identify specific toughening mechanisms. The visualization is done using CT, which is a powerful and dynamic technique, made possible by the high contrast offered by the HfO2 nanocrystals. Performing the synthesis and surface modification ourselves, gives us total control and allows to assess the effects of these parameters.