The use of tiny carriers for pharmaceuticals – such as nanoparticles for a targeted therapy – is a very old idea and long-term vision of medical research. In certain cases this dream is becoming reality: individual so-called nanocarrier systems coated with drugs are already in clinical use. It has already been demonstrated that certain pharmaceuticals can be selectively enriched in specific target tissues. Nanoparticles can also be used as carrier systems to overcome biological barriers like the blood-brain barrier. This transports specific drugs into those regions of the brain that would normally not be accessible. Nanocarrier systems can therefore be applied to improve the spatial and temporal distribution of pharmaceutical drugs in the brain to help treat and cure previously untreatable diseases. Only few studies are available on how nanoparticles unintentionally pass the blood-brain barrier or on other mechanisms such as the transport via the olfactory nerve. This hinders drawing definitive conclusions about health effects.