Extracellular fluid volume, fluid filtration rates, and the delivery of solutes to and removal of products from the interstitium all depend on the vascular permeability of the exchange vessels (capillaries and post-capillary venules), as well as haemodynamic factors such as microvascular pressure and flow. In conditions such as cancer, diabetes and arthritis, this permeability is increased and tissue homeostasis disrupted. The majority of recent studies investigating microvascular permeability have concentrated on the endothelial cell junctions, usually in cultured endothelial cells, and have described permeability changes in terms of disruption of the endothelial barrier formed by adherens junctional proteins. Recent work using single perfused capillaries in vivo in our laboratory have indicated that alterations in microvascular permeability – both increased permeability in hyperglycaemia, and reduced permeability by stabilising molecules – cannot be accounted for by alteration in the barrier proteins such as the adherens junctions, as they alter the convective permeability to water, without altering the size of the pores allowing macromolecules to penetrate the endothelial barrier. In fact the alterations can best be accounted for by changes in the barrier properties attributed to the surface coating of the endothelial cells by glycocalyx, and subsequent alteration of the fibre-matrix postulated to regulate fluid and solute permeability. The permeability properties associated with such changes are ones that are likely to contribute to alterations of vessels in diabetes (hyperglycaemia) and cancer (VEGF and Ang-1 mediated permeability changes), and the implications of these findings will be discussed.