Publisher Summary The sesquiterpenoids comprise the group of C15 compounds obtained from the vegetable essential oils, that is, the oils of the essence or distillate of plants. This chapter profiles sesquiterpenoid hydrocarbons, such as farnesenes, bisabolenes, zingiberenes and curcumenes, elemenes, santalenes, bergamotene, cuparene, cadinenes, eremophilene, eudalene, the calarane group, the azulene group, himachalenes, humulenes, caryophyllene, and related compounds. It also describes the sesquiterpenoids with an oxygen function such as farnesol, santalols, elemol, cadinols, mustakone; furanoid derivatives, such as atractylon, maaliol, eremophilone; calarane derivatives such as aristolone, drimenol; spiro compounds, such as acorenone, guaiol, and related azulenes, cedrol, widdrol, patchouli alcohol; and macrocyclic types, such as germacrone, valerenic acid, and related compounds. Methods of synthesis, general properties, typical reactions, and the stereochemical aspects of these compounds are also analyzed. α-Santonin is a lactone, the ring of which may be reversibly opened to give the corresponding acid, santoninic acid. The unsaturated ring is readily aromaticized: with hydrochloric acid, α-santonin yields desmotroposantonin. Special importance is attached to the discussion of drimenin and related compounds, lactones related to eremophilane, compounds related to the azulenes, such as guaianolides, costus lactones; abnormal guaianolides such as parthenin; and macrocarbocyclic lactones such as pyrethrosin.