Plants, fungi and corals have provided a rich source of naturally occurring compounds whose occurrence, structural elucidation, and biogenesis have proved of great interest to the organic chemist. Historically, it was the high proportion of terpenes, particularly the sesquiterpenes, in essential oils of plants that first commanded attention. In 1895 Chapman isolated a sesquiterpene from oil of hops which he distinguished from caryophyllene by the preparation of a number of derivatives and suggested the name humulene. The structure of humulene, however, remained a mystery for over fifty years. Later work showed it to be monocyclic and to possess three double bonds which were not in conjugation. The position and stereochemistry of the double bonds were then deduced by degradative experiments and analysis of n.m.r. spectroscopic data, which placed them as endocyclic and all trans. The structure of humulene was confirmed as (1) by X-ray analysis of the bis-silver nitrate adduct.