Cysteamine is a biological compound produced in the gastrointestinal tract and hypothalamus of all animals that acts on the somatotrophic axis. Cysteamine is finding increasing application in human medicine and also as a natural, in-feed growth promotant for monogastric and ruminant livestock that increases feed conversion efficiency, growth rate and leanness. It improves nutrient digestion and absorption by increasing portal-drained viscera blood flow and net portal absorption, while also reducing gastroenteropancreatic, plasma and hypothalamus concentrations of the inhibitory hormone, somatostatin (SRIF). Dietary inclusion rates required to achieve growth responses are typically about 10 times higher in ruminants than those required for pigs, but it is unclear whether ruminal breakdown of cysteamine is contributing to this difference. While short-term stimulation of growth, milk production and improved feed use efficiency are apparent, studies over longer periods are required, especially in breeding animals, due to the process of SRIF depletion being reversible. This review provides an overview of cysteamine's mode of action in improving nutrient utilisation and its application in human nutrition and health, as well as its potential use as a growth promotant in the livestock industries.