The future of medicine lies in therapeutic approaches that can specifically target sites of disease and induce a localized and effective therapeutic effect. Cell-mediated delivery, especially using cells of the circulatory repertoire such as T lymphocytes, represents an extremely attractive opportunity to transport and deliver therapeutic materials across endothelial barriers to sites of inflammation. The work presented in this Thesis explores the feasibility of using a T cell carrier to transport model nanoparticles to the central nervous system (CNS). The use of non-phagocytic cells as carriers, such as T lymphocytes, for the transport of nanomaterials essentially relies on the attachment of the therapeutic cargo on their surface. Cell-surface modification is an emerging field of research with many opportunities for therapy. This work has also tried to address a more fundamental question encountered in this field and describes a novel methodology to precisely and quantitatively determine the localization of nanomaterials on the cell carrier after surface-conjugation. Chapter 1 presents a review of the literature that covers the different approaches used for the attachment of synthetic nano- and microparticles to a variety of cells, spanning from erythrocytes, monocytes and macrophages to T lymphocytes and adult stem cells. This Chapter highlights the (bio)chemical tools that are available to modify cell surfaces with synthetic materials. These methods have been used for the attachment of a variety of synthetic materials such as polymer micron-size patches, polymer nano- and microparticles as well as lipid-based nanomaterials. An emphasis was given to therapeutic applications of the surface modified cells, which can be used to enhance systemic circulation time of nanoparticles or to mediate the transport of nanomaterials to a site of disease or in combination with cell therapy. Chapter 2 explores the use of a cell carrier to mediate the transport of nanoparticles to the CNS. Here CD4+ TEM cells were used as vehicles to deliver polymer nanoparticles across the blood-brain barrier (BBB). In fact, effective delivery of CNS active compounds across the BBB remains as one of the biggest challenges in drug delivery. Polymer model nanoparticles were attached on the surface of T cells using a maleimide-thiol covalent ligation. Nanoparticle decorated T cells were subjected to a series of functional assays to determine the influence of surface-conjugation. In particular, their ability to cross the BBB was assessed and it was demonstrated that surface-modified T cells remained functional and that they were able to transport partially their nanoparticle cargo across the BBB in vitro. The multistep extravasation of nanoparticle modified T cells across the BBB was observed for the first time by means of time-lapse live cell imaging techniques. Chapter 3 capitalizes on a method that was developed in Chapter 2 for the determination of the localization and distribution of nanoparticles on T lymphocytes after surface-conjugation. 3D-reconstruction of confocal micrograph z-stacks followed by image processing provided the distance of fluorescently labelled nanoparticles from the cell surface as function of time. The method developed previously was here challenged with a range of different nanoparticles sizes and across two different cell lines and was validated for semi-quantitative estimation of surface-conjugated versus internalized nanoparticles.