**1. Introduction**

In recent decades, an increased interest has been given to alternative medicine. In this respect, the return to herbal care is highly recommended. Indeed, plants and their therapeutically active substances have been widely used in ethno-medicine [1]. On the other hand, researches on the benefits of physiotherapy and aromatherapy, using essential oils (EOs) for healing purposes, were constantly increasing [2]. The medicinal properties of essential oils are thus widely described.

Essential oils (EOs) are products with a rather complex composition, containing volatile active ingredients. Physically, these are volatile, which differentiates them from fixed oils. They are colorless liquids with a generally strong odor and flavor [3]. They are poorly miscible with water and well soluble in oils and organic solvents [4]. Finally, the obtained oils are separated by the difference in density, generally by simple decantation [5].

According to botanists, there are approximately 800,000 to 1500,000 plant species, 10% of which contain EOs. Indeed, they have been reported to be present in about 2000 species distributed in 60 botanical families, such as Lamiaceae, Lauraceae Myrtaceae, Rutaceae, Asteraceae, Cupressaceae, Poaceae, and Zingiberaceae [6].

EOs are located in various parts of the plant (roots, fruits, seeds, flowers, leaves, bark, and wood). The biosynthesis of these compounds realizes in plant cells via various metabolic reactions, such as isopentenyl diphosphate and its isomer dimethylallyl diphosphate. The end products of EOs are terpenoid and are synthesized with a large group of enzymes called terpene synthases. They occur in the cytoplasms of plants. Then, they localize in several organs of the plant such as trichomes, epidermal cells, and, finally, the secretory pockets [7].

According to reprts, EOs are distinguished by their smells, colors, densities, and chemotypes. Therefore, each essential oil has its characteristics, fragrance, and properties. Importantly, EOs are defined by their botanical species, part of the plant, the extraction mode, and the characteristic active principle [8].

EOs are characterized with several biological activities, such as fungicide, insecticide, herbicide, and bactericide potentials. These could be used as antiseptic and antimicrobial properties [9]. Many reports have shown that these volatile compounds have antioxidant, antiviral, and antiparasitic properties. EOs are also utilized as drugs in cancer chemotherapy [10]. These bioactive molecules have been useful in dentistry for the disinfection of dental pulp and the treatment/prevention of caries [11].

In animal nutrition, essential oils have attracted the attention of nutritionists for their potential role as an alternative to growth-promoting antibiotics. In small ruminants, essential oils are characterized for their beneficial effects on the digestion and digestibility of food. More importantly, research has shown that the inclusion of essential oils at reasonable doses modulated rumen fermentation parameters such as organic matter digestibility (OMD), volatile fatty acids (VFA), and metabolizable energy (ME) [12, 13].

On the other hand, human and animal poisoning with essential oils has been reported in several research works [14, 15]. Due to the increase uses of essential oils, the number of poisonings is expected to amplify in the future. It is therefore interesting to use EOs in animals and humans with caution.

In the available chapter, we firstly defined the essential oils, by studying their extraction methods and chemical structure, as well as their main uses. Then, in a second part, we developed their pharmacological and beneficial effects in animal nutrition.
