IN THE PAST TWO DECADES, metabolic engineering has emerged as a promising mechanism for enhancing biological production of small molecules. Living organisms have long been a source of food, fuel, therapeutics, and other valuable chemicals. Early breeding techniques enabled domestication of wild plants and animals and allowed human populations to ourish. Today, novel genetic methods based on recombinant DNA technology promise to accelerate the construction and selection of highly productive organisms. In the coming decades, scientists will use these methods to engineer a sustainable existence for generations to come. ree areas of particular concern with regard to long-term sustainability are the health of the human population, the generation of energy, and the production of chemicals. Metabolic engineering has begun to address each and will continue to provide solutions in the future.