Essential oils (volatile oils, ethereal oils) are the odorous principles found in various plant parts. Depending on the plant family, essential oils may occur in specialized secretory structures such as glandular hairs (Lamiaceae), modified parenchyma cells (Piperaceae), oil-tubes called vitae (Umbelliferae), or in lysigenous or schyzogenous passages (Pinaceae). Chemical constituents of essential oils can be divided into 2 broad classes, based on their biosynthetic origin: (1) terpene derivatives, and (2) compounds formed via the shikimic acid-phenylpropanoid route. Although essential oils differ greately in their chemical constitution, they have a number of physical properties in common. They possess characteristic odors, they are characterized by high refractive indices, most of them are optically active, and their specific rotation is often a valuable diagnostic property. They are immiscible with water, but they are sufficiently soluble to impart their odor to water. The aromatic waters are dependent on this slight solubility. However, they are soluble in ehter, alcohol and most organic solvents. Essential oils are usually obtained by distillation of the plant parts containing oil. Three types of distillation are used by industrial firms: (1) water, (2) water and steam, and (3) direct steam. In the parfume industry, most of the modern essential oil production is accomplished by extraction, using solvent system based on such volatile solvents as petroleum ether or benzene. The process known as enfleurage was formerly used extensively in the production of parfumes and pomades and was based on absorption of essential oil by purified fat. The volatile oil drugs and the separated oils are most commonly used for flavoring purposes, but some of them possess additional therapeutic properties (antimicrobial, antifungal, anthelmintic, carminative, expectorant, diuretic, antiphlogistic etc.). In addition to their pharmaceutic uses, essential oils are employed widely as flavors for foods and confections and in the spice, parfume and cosmetic trades.