Transport across the blood–brain barrier is the dominant mechanism for removal of water and CO2 from brain parenchyma (for discussion and references see [4]). Molecules less lipid soluble or somewhat larger than H2O need specific transporters in the endothelial cell membranes of the barrier, e.g. for glucose GLUT1, which is found in both luminal and abluminal membranes. Transporters are present for a large number of substances [20, 31, 46, 55–58] (see Sect. 4.2). Certain larger solutes, e.g. insulin [59], transferrin [60, 61] and β-amyloid [62], may be transported across the blood–brain barrier by transcytosis [36, 63, 64] (see Sect. 4.3).