Isoprenoids produced by bacteria are of particular interest as they encompass huge structural and functional diversity. A rapidly increasing number of bacterial derived isoprenoids have been reported in recent years, as many of the genes and biosynthetic gene clusters are responsible for their biosynthesis. Polyprenyl chains, synthesized by the condensation of isopentenyl diphosphate units, serve as the substrates for cyclases and subsequent tailoring enzymes. It is these enzymes, particularly the cyclases, which are responsible for the structural diversity of this chemical class. Recent studies have revealed novel insights into isoprenoid biosynthesis, and in several cases enzymatic mechanisms have been redefined.