Betaine, a donor of labile methyl groups, can spare choline and methionine but cannot replace these compounds in poultry diets. Betaine is synthesized from choline by choline oxidase and it can donate methyl groups to homocysteine to form methionine. Physiologically, betaine is one of several compounds used by cells to regulate osmotic pressure. Among the potential benefits of its inclusion in poultry feeds are sparing choline, carcass fat reduction and aiding cell osmoregulation. Some feed ingredients are natural sources of betaine per se. This review considers the metabolism, functions and applications of betaine in poultry.