Betaine (tri-methylglycine) is a natural compound present in bacteria, plant and animal cells. It is an important nutrient in human and animal diets and exerts functions in osmolality regulation, maintenance of cell function as a chaperone and one-carbon metabolism as a methyl donor. Betaine in the animal body can be synthesized endogenously by choline oxidation or absorbed from the diet. As a methyl donor, betaine is catalyzed by betaine-homocysteine methyltransferase and donates a methyl group to homocysteine for re-synthesizing methionine, primarily in liver. This potentially spares choline or methionine as methyl donors, and promotes the production of S-adenosylmethionine, a universal methyl donor. As an organic osmolyte, betaine regulates cell volume and organizes water structure in a cell, stabilizing cellular proteins, especially under stress conditions. Therefore, betaine has numerous functions in an animal’s body. In laboratory animals, betaine supplementation is reported to alleviate hepatic fat accumulation and other symptoms associated with alcohol or non-alcohol related liver injury. In coccdia-infected poultry, betaine stabilizes intestinal structure and maintains gut health and function. In swine and poultry, betaine is used as a “carcass modifier” to increase lean tissue synthesis. In ruminants, limited research suggest that supplementation of betaine influences rumen fermentation, improves nutrient digestibility, and increases milk yield during lactation. Because it functions as a methyl donor and an organic osmolyte, betaine has the potential to be utilized in dairy cattle diet during the transition period and summer to improve hepatic health and improve stress responses. However, convincing evidence is still limited and extensive research is warranted.