<p> Terpenoid cyclases catalyse remarkably complex cyclisation cascades that are initiated by the formation of a highly reactive carbocation in a polyisoprene substrate [<cite idref="PUB00010630"/>]. The pathways of monoterpene, sesquiterpene, and diterpene biosynthesis are conveniently divided into several stages [<cite idref="PUB00010631"/>]. The first encompasses the synthesis of isopentenyl diphosphate, isomerisation to dimethylallyl diphosphate, prenyltransferase-catalysed condensation of these two C5-units to geranyl diphosphate (GDP), and the subsequent 1'-4 additions of isopentenyl diphosphate to generate farnesyl (FDP) and geranylgeranyl (GGDP) diphosphate. In the second stage, the prenyl diphosphates undergo a range of cyclisations based on variations on the same mechanistic theme to produce the parent skeletons of each class. Thus, GDP (C10) gives rise to monoterpenes, FDP (C15) to sesquiterpenes, and GGDP (C20) to diterpenes. These transformations catalysed by the terpenoid synthases (cyclases) may be followed by a variety of redox modifications of the parent skeletal types to produce the many thousands of different terpenoid metabolites of the essential oils, turpentines, and resins of plant origin. Terpenoid synthases enzymes provide a template for binding and stabilising the flexible substrate in the precise orientation required for catalysis, trigger carbocation formation, chaperone the conformations of the reactive carbocation intermediates through a unique cyclisation sequence, and sequester and stabilise carbocations from premature quenching [<cite idref="PUB00010630"/>].</p>