Nanometer-sized materials are used in nanomedicine to deliver drugs more efficiently to their site of action. In order to improve their efficacy, a better understanding of how cells interact with nanometer-sized materials is required. Our research is focused on characterizing the molecular details of the early interactions of nanometer-sized materials at the cell membrane, and the subsequent mechanisms of uptake and intracellular trafficking. To this end, we combine classic transport studies with inhibitors and RNA interference to genetic screening and proteomic-based methods to characterize the mechanisms by which nanoparticles are internalized by cells. In further studies, we developed advanced in vitro models more closely resembling the in vivo environment for our studies, including endothelial cell barriers and precision cut tissue slices from different organs. In this talk, I will highlight our findings on how cells interact with and process nanometer-sized materials and present first results for the improvement of the drug-delivery applied in nanomedicine.