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The aims of the PhD project were to understand the mechanisms of cellular uptake as well as the intracellular biostability of oxidised functionalised multi-walled carbon nanotubes (f- MWNTs). Firstly, high resolution transmission electron microscopy (HR TEM) imaging and 3D electron tomography were applied to study the pathways of f-MWNTs into non-phagocytic cells, and more specifically, to study the interaction between f-MWNTs and the cell membrane. After exposing epithelial cells to f-MWNTs for 24 hours, two distinct uptake processes by which NH3+ f-MWNTs can enter epithelial cells were observed. Then, the study was complemented using a combination of TEM imaging and coarse-grained molecular dynamics simulations, to provide insight into the interaction of f-MWNTs with cell membranes and the effect of surface charge on this interaction. Secondly, the question of whether f-MWNTs can be degraded by the body’s own defence mechanisms was addressed. HR TEM techniques were used to assess the graphitic structure and morphology of f-MWNTs injected into the murine brain and after exposure to human monocyte-derived macrophages (HMMs), with the aim of understanding the mechanisms underlying the degradation process. F-MWNTs were found to have a reduced biostability in the brain tissue and in the HMMs. Inside the brain tissue, the degradation occurred rapidly with signs of advanced f-MWNT degradation present after 2 days exposure. In the HMMs, f-MWNT walls were found to delaminate from individual f-MWNTs inside lysosomes after 24 hours exposure. Similar events were observed after 14 days exposure in the cell cytoplasm inside the HMMs. Furthermore, a loss of the graphitic structure was observed. By using scanning TEM electron energy loss spectroscopy (STEM EELS) to compare the near-edge structure of the carbon K-edge prior to, and post injection, graphitic f-MWNTs could be distinguished from the graphitic debris and the amorphous cell background. The combination of HR TEM and STEM EELS techniques provided information about the individual steps, leading to the disintegration of the f-MWNTs, and the morphology of the degradation debris. In order to study the effect of the functionalisation on the biostability, the study was repeated using pristine MWNTs and HMMs. No signs of degradation of the pristine MWNTs were observed after 14 days exposure to the HMMs.