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 giving rise to not only a large super-family of ~7,000 diterpenoids, but 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 some discussion also is provided of the important subsequent elaboration in those few cases where molecular genetic information is available. In addition, the array of biological activity providing the selective pressure driving the observed gene family expansion and diversification, along with biosynthetic gene clustering, will be discussed as well.