[en] Isoprenoids form an extensive group of natural products involved in a number of important biological processes. Their biosynthesis proceeds through sequential 1'-4 condensations of isopentenyl diphosphate (C(5)) with an allylic acceptor, the first of which is dimethylallyl diphosphate (C(5)). The reactions leading to the production of geranyl diphosphate (C(10)), farnesyl diphosphate (C(15)) and geranylgeranyl diphosphate (C(20)), which are the precursors of mono-, sesqui- and diterpenes, respectively, are catalyzed by a group of highly conserved enzymes known as short-chain isoprenyl diphosphate synthases, or prenyltransferases. In recent years, the sequences of many new prenyltransferases have become available, including those of several plant and animal geranyl diphosphate synthases, revealing novel mechanisms of product chain-length selectivity and an intricate evolutionary path from a putative common ancestor. Finally, there is considerable interest in designing inhibitors specific to short-chain prenyltransferases, for the purpose of developing new drugs or pesticides that target the isoprenoid biosynthetic pathway.