Monoterpenes are important renewable resources for the perfume and flavour industry but the pathways and enzymology of their degradation by microorganisms are not well documented. Until recently the acyclic monoterpene alcohols, (+)-camphor and the isomers of limonene were the only compounds for which significant sections of catabolic pathways and associated enzymology had been reported. In this paper recent developments in our understanding of the enzymology of ring cleavage by microorganisms capable of growth with 1,8-cineole and a-pinene are described. 1,8-Cineole has the carbocyclic skeleton of a monocyclic monoterpene with the added complication of an internal ether linkage. Ring hydroxylation strategy and biological Baeyer-Villiger oxygenation lead to an efficient method for cleaving the ether linkage, et-Pinene is an unsaturated bicyclic monoterpene hydrocarbon. At least two catabolic pathways exist. Information concerning one of them, in which a-pinene may be initially converted into limonene, is rudimentary. The other involves attack at the double bond resulting in formation of a-pinene epoxide. Ring cleavage is then catalysed by a novel lyase that requires no additional components and breaks both carbocyclic rings in a concerted manner. . 5 CO(~ ~ S 9 O ~ 4[ 4 CO