Environmental conditions play a key role in poultry husbandry. The interaction between environment and the effects of essentials oils should be more researched. Detailed description of the hygienic conditions is necessary for better interpretation of the experimental results and variations. The anticoccidial and immunomodulatory properties of essential oils in poultry have gained interest and require more in-depth research. The metabolic fates of essential oils in poultry should be studied, and the corresponding analytical methods should be established to track the active compounds and their metabolites. Research on the interactions among individual essential oils and with other categories of feed additives should be explored for identifying practical applications.