The blood-brain barrier (BBB) is formed by epithelial-like high resistance tight junctions within the endothelium of capillaries perfusing the vertebrate brain. Because of the presence of the BBB, circulating molecules gain access to brain cells only via one of two processes: (i) lipid-mediated transport of small molecules through the BBB by free diffusion, or (ii) catalyzed transport. The latter includes carrier-mediated transport processes for low molecular weight nutrients and water soluble vitamins or receptor-mediated transport for circulating peptides (e.g., insulin), plasma proteins (e.g., transferrin), or viruses. While BBB permeability, per se, is controlled by the biochemical properties of the plasma membranes of the capillary endothelial cells, overall brain microvascular biology is a function of the paracrine interactions between the capillary endothelium and the other two major cells comprising the microcirculation of brain, i.e., the capillary pericyte, which shares the basement membrane with the endothelial cell, and the astrocyte foot process, which invests 99% of the abluminal surface of the capillary basement membrane in brain. Microvascular functions frequently ascribed to the capillary endothelium are actually executed by either the capillary pericyte or the capillary astrocyte foot process. With respect to BBB methodology, there are a variety of in vivo methods for studying biological transport across this important membrane. The classical physiologic techniques may now be correlated with modern biochemical and molecular biological approaches using freshly isolated animal or human brain capillaries. Isolated brain capillary endothelial cells can also be grown in tissue culture to form an 'in vitro BBB' model. However, BBB research cannot be performed using only the in vitro BBB model, but rather it is necessary to correlate observations made with the in vitro BBB model with in vivo studies.