The structures and mechanism of action of many terpene cyclases are known, but there are no structures of diterpene cyclases. Here, we propose structural models based on bioinformatics, sitedirected mutagenesis, domain swapping, enzyme inhibition and spectroscopy that help explain the nature of diterpene cyclase structure, function, and evolution. Bacterial diterpene cyclases contain ～20 α-helices and the same conserved “QW” and DxDD motifs as in triterpene cyclases, indicating the presence of a βγ barrel structure. Plant diterpene cyclases have a similar catalytic motif and βγ-domain structure together with a third, α-domain, forming an αβγ structure, and in H+-initiated cyclases, there is an EDxxD-like Mg2+/diphosphate binding motif located in the γ- domain. The results support a new view of terpene cyclase structure and function and suggest evolution from ancient (βγ) bacterial triterpene cyclases to (βγ) bacterial and thence to (αβγ) plant