Ruminant livestock systems contribute significantly to emission of methane, a potent greenhouse gas as they waste a portion of the ingested energy (2–15%) as methane and a large proportion (75–95%) of the ingested nitrogen as ammonia. Recently, numerous researches have been conducted to evaluate plant secondary metabolites, including essential oils (EO), as natural feed additives in ruminant nutrition and to exploit their potential to improve rumen fermentation efficiency. Essential oils appeared to be very promising compounds as they selectively reduced methane production and protein breakdown in both in vitro and in vivo studies. However, in some studies, the use of EO as feed additives was accompanied with decreased feed degradability and lowered volatile fatty acid. These adverse effects could be attributed to their broad and often non-specific antimicrobial activities within the rumen. Future research should be directed to identification of the active and useful EO compounds, optimization of EO doses, and use of a whole-farm approach with a focus on animal welfare, performance and economic benefits. Display omitted •Ruminants contributes 16–25% to the global greenhouse gases emissions.•Decrease methane emission and nitrogen excretion from ruminant livestock industry is urgently needed.•Essential oils have been shown to be promising feed additives in mitigating methane and ammonia emissions.•Essential oils have showed inconsistent results about feed degradability and VFA production.•The mode of action and activities of essential oils on rumen microbiome remain poorly understood.