This review covers recent advances in the chemistry and enzymology of squalene cyclase and oxidosqualene cyclase. The enzymatic cyclizations of squalene and oxidosqualene are the most remarkable steps in the biosynthesis of sterols and triterpenes. The polyenes are converted to various polycyclic triterpenes by different enzyme systems employing only small modification of the active-site. Recent crystallographic and structure-based mutagenesis studies as well as utilization of chemically synthesized active-site probes have begun to reveal intimate structural details of the enzyme-templated cyclization reactions. 126 References are cited.