The diterpenoids are classically defined by their composition--four isoprenyl units (20 carbons)--and are generally derived from E,E,E-geranylgeranyl diphosphate (GGPP). Such metabolism seems to be ancient and has been extensively diversified, with ∼12,000 diterpenoid natural products known. Particularly notable are the gibberellin phytohormones, whose requisite biosynthesis has provided a genetic reservoir that gave rise to not only a large superfamily of ∼7,000 diterpenoids but also, to some degree, all plant terpenoid natural products. This review focuses on the diterpenoids, particularly the defining biosynthetic characteristics of the major superfamilies defined by the cyclization and/or rearrangement of GGPP catalyzed by diterpene synthases/cyclases, although it also includes some discussion of the important subsequent elaboration in the few cases where sufficient molecular genetic information is available. It additionally addresses the array of biological activity providing the selective pressures that drive the observed gene family expansion and diversification, along with biosynthetic gene clustering.