Due to the ban of antibiotic growth promoters, the use of essential oils in animal husbandry has been very much developed. In poultry feeding, essential oils produced from thyme, oregano and rosemary are the most studied. A noteworthy characteristic of essential oils is that the balance between the different active principles can be altered from one oil to another. Depending on the research that has been done, the effects of these essential oils can change a lot. Concerning thyme essential oil, guaranteeing a significant effect on growth performances is difficult; however, there is a tendency for a higher daily weight gain. As for oregano essential oil, food conversion ratio and growth performances are improved, as well as laying rates and feed intake for hens. Rosemary essential oil also seems to improve food conversion ratio. However, when used at excessive doses, essential oils can have adverse effects (an increase of feed conversion ratio in broilers and fall of global weight gain). But, on the basis that a broiler eats 0.5 mg of essential oil per kilogram of live weight per day, the toxic risk is very low, both for the broiler and for humans.