The chapter describes separately the biochemistry, neuroanatomy, functional role and clinical significance of noradrenaline (NA), dopamine (DA), and the indoleamine serotonin (5-hydroxy-tryptamine; 5HT) neurons. Noradrenaline (NA) is one of the most important mammal catecholamines chemically characterized by a benzene ring and a catechol nucleus. Stimulation of sympathetic nerves leads to release of a substance, originally called sympathin, subsequently identified as NA. DA was discovered as an independent neurotransmitter in the brain. Serotonin or 5-hydroxy-tryptamine (5-HT), initially identified in peripheral tissues, was first detected in the mammalian central nervous system 40 years ago. Most of the different 5-HT receptor subtypes are located on the postsynaptic targets of serotoninergic neurons. DA and NA nuclei, pathways, receptors, and transmitter release are also discussed in the chapter. The chapter presents the functional role and clinical significance of brain DA, NA, and 5-HT neurons.