Publisher Summary This chapter discusses the transport of amino acid across the blood-brain barrier (BBB). Amino acids serve multiple roles in brain as neurotransmitters, neurotransmitter precursors, and building blocks of peptides and proteins. Their levels in brain are closely regulated in part by controlled transport across the BBB. The BBB is a system of tissue sites including brain vascular endothelial cells, choroid plexus epithelial cells and arachnoid membrane, which together restrict and regulate the exchange of polar solutes between plasma and brain extracellular fluid. The physical barrier at each site is formed by a single layer of cells joined together by multiple bands of tight junctions. These junctions essentially seal adjacent cells together and block the aqueous paracellular diffusion pathway. In the absence of paracellular diffusion, polar solutes, such as amino acids, are forced to cross the BBB either by passive diffusion through lipoid BBB membranes or by carrier-mediated transport across the membranes via one of more than 12 BBB amino acid transport proteins.