Abstract: Essential Oils (EO) are volatile aromatic compounds extracted from whole plants and are secondary metabolites usually made up of terpenoids and phenylpropanoids. Plant EO has antimicrobial properties, which can be effective against undesirable rumen microbes. Therefore, recently it has been great interests among nutritionists and rumen microbiologists to exploit EO as natural feed additives to improve rumen fermentation such as volatile fatty acids production, inhibition of methanogenesis, improvement in protein metabolism and efficiency of feed utilization and increasing conjugated linoleic acids in ruminant derived foods. Different types of EO from a wide range of herbs and spices have been identified to have potential for rumen manipulations and enhancing animal productivity as alternatives to chemical feed additives. However, their effectiveness in ruminant production has not been proved to be consistent and conclusive. There are varying reports of EO on rumen fermentation, rumen microbiota and ruminant performance depending upon the dose, chemical structures of EO, feed composition and animal physiology, which have not always been adequately described in the literature. The comprehensive research based on individual components of EO, physiological status of animals, nutrient composition of diets and their effects on rumen microbial ecosystem and metabolism of EO will be required to obtain consistent beneficial effects.