EOs are complex mixtures of volatile compounds extracted from a large number of plants. In general they represent a small fraction of plant composition (less than 5% of the vegetal dry matter) and comprise mainly hydrocarbon terpenes (isoprenes) and terpenoids. The first compounds are monoterpenes (they have 10 carbon atoms and represent more than 80% of EOs composition) and sesquiterpenes (they have 15 carbon atoms). They could present hydrocarbon acyclic structures, so as mono-, bi- or tricyclic structures. The second ones, also called isoprenoids. They are oxygenated derivatives of hydrocarbon terpenes such as, alcohols, aldehydes, ketones, acids, phenols, ethers and esters