**1. Introduction**

Essential oil (EO) is a secondary metabolite synthesized by medicinal and aromatic plants. It represents less than 5% of the total plant composition. Over 3000 types of EO have been identified, but only 300 were found to be economical [1, 2].

EO is volatile, generally colorless, and liquid at room temperature. It is highly soluble in organic solvents, alcohol, and fixed oils but sparingly soluble in water. It has very high optical activity, a variable refractive index, and sometimes a distinctive taste. In addition, essential oils have a characteristic odor, that is responsible for the fragrance specific to the aromatic plant. Chemically, EO components can be divided into terpene compounds and aromatic compounds. It is a mixture of bi-active chemical components such as terpenoids, terpenes, and phenolic compounds. They are made up of terpene compounds, acids, alcohols, esters, aldehydes, ketone epoxides, sulfides, and amines [3, 4].

They are synthesized by the majority of plant organs, in particular flowers, buds, leaves, seeds, stems, and fruits. These EOs can be stored in epidermal cells, cavities, and the secretory cells of glandular trichomes.

EOs are known for their biological activity, notably their antioxidant, antifungal, antimicrobial, antiviral, antiparasitic, antimycotic, and insecticidal properties [5, 6].

Several extraction techniques are used; Hydrodistillation and steam extraction are the oldest, simplest, and most commonly used methods. Other extraction methods can also be used: cold pressing, especially applied to rose.

The EO extraction method generally depends on the botanical material used. It is one of the main factors determining EO quality. An inappropriate extraction procedure can damage or alter the chemical composition of EO. This results in a loss of bio-activity and natural characteristics. In the most serious cases, this is accompanied by discoloration, an unpleasant odor or flavor, and physical changes such as increased viscosity [7].

The principle of EO extraction is relatively straightforward. However, the process chosen can have a significant effect on the yield and quality of the distillate obtained [8]. Various processes have therefore been developed to obtain maximum yields of EO with a chemical state as close as possible to their native structure.

According to the European Pharmacopeia, an essential oil can be obtained by steam distillation, distillation, or a mechanical process. Other processes include supercritical CO2 extraction, volatile organic solvent extraction, microwave extraction, and ultrasonic extraction. The aim of this chapter is to present an overview of the various extraction methods.
