A terpene is a naturally occuring organic compound with the general formula (C 5 H 8 ) n . Until recently this definition was used rather strictly. The term terpenoid , refering to related compounds containing oxygen, has fallen into disuse, and both classes are now known as terpenes. The terpene share a common general formula and often have structures related to the diene isoprene: (2-methyl-1,3-butadiene) = isoprene, CH 2 C=CH 3 CH=CH 2 . The number of isoprene units serves as the most common classification system for the large number of known terpenes. The German chemist Otto Wallach received the Nobel Prize for chemistry in 1910 for his extensive studies of terpene chemistry. Many terpenes are commercially valuable. For example, the monoterpenes and ssquiterpenes are major comstituents of many essential oils prized as perfumes and flavors. There are important relationship between the higher terpenes and steroids, carotenoids and vitamins. The carotenoids β-carotine (a tetrape