Ionophores were first used as feed additives during the 1970's. Today, approximately 67 % of the broilers and 95 % of feedlot cattle in the United States receive an ionophore for a significant portion of the feeding period. More than 1.4 X 106 metric tons of feed are saved annually by beef cattle feeders because of the feed efficiency improvement of ionophores. Actions of ionophores are primarily directed at ruminal bacteria and are limited to the gastrointestinal tract. Production efficiency of cattle is improved by three mechanisms: increased energy availability from the diet; reduced degradation of dietary protein; and decreased morbidity and mortality associated with digestive disturbances in the rumen. Ionophores offer significant potential to lactating dairy cattle through the same mechanisms. Milk production is increased in early lactation when energy intake is limiting. Feed savings are realized in mid and late lactation and during the dry period. Yield of milk protein is increased while yield of fat is unchanged. Ionophores reduce the incidence and severity of bloat and ketosis and may help control acidosis. As a consequence of feeding ionophores, the cost to produce milk is reduced because of increased milk production per unit of dry matter consumed and from reduced veterinary costs associated with reduced incidence of digestive and metabolic disease. Body condition of cows is improved throughout lactation with ionophores. Ionophores are friendly to the environment. Less feed is required per unit of edible product that is produced. Less methane is produced during fermentation, and total tract digestibilities of organic matter and protein are increased resulting in less excreta. The ionophores, monensin and lasalocid, are used successfully by dairymen in several countries around the world. However, no ionophore is currently approved for use in lactating dairy cows in the United States.