Abietadiene synthase from Abies grandis (AgAS) has served as a model for investigation of diterpene synthase activity, and here we report its crystal structure at 2.3 Å resolution. This bifunctional enzyme catalyzes both class I (ionization-initiated), as well as class II (protonation initiated) cyclization reactions, and is composed of three α-helical domains, designated α, β, and γ, wherein the class I active site falls within the C-terminal α domain and the class II active site lies between the N-terminal γβ domains. The structure not only clarifies the evolutionary origins of diterpene synthases, but also provides insights into the enzymatic structure-function relationships underlying both a recently discovered regulatory mechanism, as well as catalyzed reactions. In particular, exchange of aliphatic and aliphatic hydroxyl side chains at a single amino acid position can have dramatic effect on product outcome in diterpene synthases. The location of the relevant residue differs slightly between various enzymes, and the effect of side chain chemistry has not been further explored. Here we show that the ability of these single residue changes to affect product outcome is specific for both active site location and side chain chemical composition, as well as further demonstrate a direct interaction between the relevant residue and carbocation intermediate. In addition, other investigations involved in this thesis includes the impact of the secondary position in the active site to product outcome, the conservation pattern of a putative second terpene synthase divalent metal binding motif in plants, as well as functional characterization of wheat diterpene synthases.