Betaine (trimethylglycine) is a nontoxic methylamine compound contains the amino acid glycine with three chemically reactive methyl groups. This review gives an overview of the roles of betaine as a feed additive on the performance, carcass, meat quality in livestock production. Biological functions of betaine are defined as an osmolyte, methyl donor, and carcass modifier. As an osmolyte, betaine regulates the cellular hydration status and alleviates osmotic stress. As a methyl donor, betaine plays an important role in methylation reactions. Therefore, betaine may partly replace other methyl donors such as choline and methionine in the diet. Betaine is also supposed to play a significant role in lipid metabolism and has lipotropic effects also participating in the protein synthesis. A review of the results of this study suggests that betaine supplementation was effective to improve the average daily gain significantly, increased feed consumption and improved efficiency of food utilization of ruminants and monogastric animals. Betaine has been characterized as a carcass modifier by increasing the lean carcass percentage and decrease fat percentage by reducing lipid deposition. In conclusion, this study implies that dietary betaine supplementation was efficacious on growth performance and carcass characteristics of livestock.