This thesis describes functional characterization of three oxidosqualene cyclase genes (At1g78955, At3g45130, and At4g15340 ) from the model plant Arabidopsis thaliana that encode enzymes with novel catalytic functions. Oxidosqualene cyclases are a family of membrane proteins that convert the acyclic substrate oxidosqualene into polycyclic products with many chiral centers. The complex mechanistic pathways and relevant catalytic motifs can be elucidated through judicious applications of mutagenesis, heterologous expression in combination with a genome mining approach, and protein modeling. Functional characterization of oxidosqualene cyclases allows improved understanding on how these proteins guide catalytic reactions and how protein-substrate interactions affect the reaction outcome, as well as identification of triterpenes with novel structures and stereochemistry. This work describes characterization of Arabidopsis oxidosqualene cyclases, including the first plant lanosterol synthase (LSS1), an enzyme with novel catalytic motifs different from those previously observed in animal, fungal, and trypanosomal lanosterol synthases, establishing that plant lanosterol synthases comprise a catalytically distinct class of lanosterol synthases. Phylogenetic analysis reveals that lanosterol synthases are broadly distributed in eudicots but evolved independently from those in animals and fungi. Discovery of plant lanosterol synthase also suggests lanosterol as precursor for plant 4,4-dimethyl-Delta8 sterols. Additional mutagenesis experiments on Arabidopsis lanosterol synthase (Asn477His and Va1481Ile) allowed for introduction of cycloartenol activity in a lanosterol synthase background, providing the best example of engineered biosynthesis of cyclopropyl structures known to date. This thesis also describes the first enzyme (camelliol C synthase, CAMS1) that efficiently blocks B-ring formation to make a monocyclic triterpene camelliol C. Phylogenetic analysis reveals that this cyclase has evolved from enzymes that generate pentacycles, and sequence comparison between oxidosqualene cyclases with different catalytic functions allowed for identification of key residues that increases steric bulk in the active site to promote monocycle formation. Finally, this thesis describes an enzyme arabidiol synthase (PEN1) that produces the tricyclic triterpene diol arabidiol. Analysis of the arabidiol structure and characterization of numerous minor products of arabidiol synthase, including several novel compounds, resulted in formulation of a general rule for water addition in triterpene biosynthesis and an explanation for the domination of deprotonation over water addition in triterpene biosynthesis.