Cholesterol and fatty acid moieties are the most abundant lipid species in the brain. The blood brain barrier (BBB), formed by tightly juxtaposed endothelial cells, regulates the accessibility of circulating lipids to brain cells. In physiological conditions lipoprotein cholesterol cannot enter the BBB. Cholesterol necessary for myelination and other functions (for example, synthesis of oxysterols and neuroactive steroids) is synthesized in situ by astrocytes and oligodendrocytes (not shown). In contrast, 24-hydroxycholesterol can cross the BBB, thus entering the systemic circulation. Fatty acids, the building blocks of many specialized lipids, derive from blood (albumin-associated free fatty acids) or are synthesized in situ. Notably, in brain cells the fraction of fatty acids subject to b-oxidation is rather small.