Ionophores are feed additives that alter rumen microbial populations through ion transfer across cell membranes. Although ionophores have been used widely in the ruminants nutrition for improved feed efficiency and control of coccidiosis. Ionophores have been studied regarding several potential effects on ruminant animals, including influences on health, meat production, milk production and reproduction. This review will provide practitioners with relevant references in the published literature regarding ionophore use in ruminants' nutrition. It should also give some guidance as to what effects might be anticipated with the use of ionophores in ruminant animals.