Microorganisms synthesize many useful compounds and humans have exploited their metabolic versatility for thousands of years. e discovery of the rst microbial fermentation processes in the nineteenth century was the starting point for the development of industrial biotechnology. Biotechnologies introduced during the rst half of the twentieth century allowed the mass production of citric acid and penicillin through the fermentation of sugars by specic strains of molds. At the beginning, microbial production of molecules depended entirely upon the exploitation of native biosynthetic pathways in microbial organisms. Microbial strains producing a desired compound were rst identied by screening and production levels were then optimized using classical strain improvement strategies involving chemical mutagenesis and selection. While strain selection and screening sometimes did yield impressive increases in production levels, the improvements were not the result of directed engineering and the mechanism of the improvements remained obscure. As recombinant DNA technologies developed, engineering of specic metabolic traits became possible. By the 1980s it was possible to insert or delete enzymes in a microbial genome, and in one of the rst examples of this technology the production of cellular carbon from methanol for animal feed was enhanced by replacing the glutamate synthase gene from the bacterium Methylophilus methylotrophus with a glutamate dehydrogenase from E. coli.1