Essential or volatile oils form a group of organic compounds with a pleasant odor or taste, occurring in many plant species, especially family Lauraceae (Laurel family) (Lauraceae 1); family Myrtaceae (Myrtle family) (Myrtaceae 1); family Umbelliferae (= Apiaceae) (Parsley family) (Umbelliferae 1); family Labiatae (= Lamiaceae) (Mint family) (Labiatae 1); and family Compositae (= Asteraceae) (Composite family) (Compositae 1).

Essential or volatile oils differ from fixed or fatty oils because they evaporate on the air. They contain a great variety of terpenes and terpenoids, all derived from isoprene. They are produced in glands or glandular cells and occur especially in flowers, fruits, leaves and stems. In some cases, like in mustard (Brassica alba and Brassica nigra) and in horse-radish, Armoracia rusticana (see Raphanus sativus), the oil is formed as a result of enzymatic activity in damaged cells.

Essential oils can be divided into three groups: perfume oils, used in perfume, soap and detergents; wood oils, especially used in the pharmaceutical and lacquer industry; and tasty or flavouring oils, which are used in foods, drinks and candies.

Examples of perfume oil producing plants are:
- sweet flag, Acorus calamus (see Xanthosoma sagittifolium);
- ylang-ylang, Cananga odorata;
- rosemary, Rosmarinus officinalis;
- sage, Salvia officinalis;
- lavendel (Lavandula officinalis)
- thyme, Thymus vulgaris;
- patchouli, Pogestemon patchouli;
- sour, bitter or Seville orange, Citrus aurantium, (yields bergamot oil);
- citronella grass, Cymbopogon nardus (see Cymbopogon sp.);
- rosha grass, Cymbopogon martini (see Cymbopogon sp.).

In cosmetics much use is also made of oil from sandal wood, Santalum album, and piment, Pimenta dioica. Moreover, essential oils are obtained from many flowers like roses, hyacinths, lavender, pelargonium, jasmine, daisies, gardenia and carnations.

Among the most important wood oils are camphor from Cinnamomum camphora (see Cinnamomum zeylanicum), and eucalyptus oil from Eucalyptus globulus (see Eucalyptus sp.). These oils have many applications in the pharmaceutical, perfume and lacquer industry. Cajeput oil from Melaleuca leucadendron and Melaleuca quinquinervia (see for both Eucalyptus globulus) is used medicinally.

Tasty or flavouring oils are obtained from:
- peppermint, Mentha piperita;
- spearmint, Mentha spicata;
- anise, Pimpinella anisum;
- clove, Eugenia caryophyllus, (Essential oils 1);
- nutmeg, Myristica fragrans;
- cinnamon, Cinnamomum zeylanicum;
- celery, Apium graveolens;
- caraway, Carum carvi;
- lemon, Citrus limon
- lime, Citrus aurantifolia;
- hop, Humulus lupulus.

Many kitchen herbs and spices owe their fragrance to essential oils as well, see also Kitchen herbs and Spices and flavourings.