Terpenes are the primary constituents of essential oils. In general, pure essential oils can be subdivided into two distinct groups of chemical constituents; hydrocarbons which are made up almost exclusively of terpenes (monoterpenes, sesquiterpenes, and diterpenes), and the oxygenated compounds which are mainly esters, aldehydes, ketones, alcohols, phenols, and oxides. Terpenes are the primary components of essential oils of many types of medicinal plants and flowers. In addition, terpenes are also major biosynthetic building blocks with nearly every living creature. Steroids, for example, are derivatives of the triterpene squalene.