Isoprenoids are ubiquitous in living organisms. Although they possess diverse structures and biological functions, all are synthesized from two precursors, isopentenyl diphosphate (IPP) and its isomer, dimethylallyl diphosphate (DMAPP). These precursors are biosynthesized through the mevalonate pathway. This chapter concentrates on the mevalonate pathway of isoprenoid biosynthesis, especially in bacteria and archaea. The mevalonate pathway genes, which are often clustered on bacterial genomes, are illustrated. The distribution, properties, and reaction mechanisms of the two major enzymes involved, 3-hydroxy-3-methylglutaryl (HMG)-coenzyme A (CoA) reductases and IPP isomerases, are examined. Isoprenoid biosynthetic gene clusters from four actinomycete strains are also described.