For many years, ruminant nutritionists and microbiologists have been interested in manipulating the microbial ecosystem of the rumen to improve production efficiency by domestic ruminants. Antibiotic ionophores have been used successfully for this purpose. However, the use of antibiotics in animal feeds has been banned in the European Union since January 2006 due to the risk of spreading antibiotic resistance. For this reason, scientists have become interested in evaluating other alternatives to control specific microbial populations to modulate rumen fermentation. Dietary supplements of live yeast preparations, based on Saccharomyces cerevisiae, have been reported to improve health and productivity of ruminants. In contrast to antimicrobial agents, live yeasts offer a natural alternative to manipulate animal performance. This review discusses the modes of action of live yeasts in rumen ecosystem and their subsequent effects on animal performance.