The aim of my thesis was to try to identify novel pathways for trafficking molecules into the brain through the Blood Cerebrospinal Fluid Barrier and the central technology for identifying novel receptor-mediated transport pathways has been to use “in vivo phage display”. The first aim of the thesis was to develop the in vivo model to study the transport of phage across the BCSFB. The interesting candidate phage are those which bind to a transport receptor on the choroid plexus epithelium, and which are then passaged into the CSF. The other aim of the thesis was to implement an in vitro model of the BCSFB using primary epithelial cells obtained from the dissected rat choroid plexus. The in vitro model mimicking the BCSFB was implemented and validated with success. The candidates resulting from the in vivo panning experiments were then tested although unfortunately we were not able to show convincing transport of our reformatted IgGs across the choroid plexus epithelium in vitro. The reasons for this are discussed.