With the development of recombinant deoxyribonucleic acid (DNA) technology, the metabolic potentials of microorganisms are being explored and harnessed in a vari- ety of new ways. Today, genetically modified microorganisms (GMMs) have found applications in human health, agriculture, and bioremediation and in industries such as food, paper, and textiles. Genetic engineering offers the advantages over traditional methods of increasing molecular diversity and improving chemical selectivity. In addi- tion, genetic engineering offers sufficient supplies of desired products, cheaper prod- uct production, and safe handling of otherwise dangerous agents. This chapter delineates several molecular tools and strategies to engineer microorganisms; the advantages and limitations of the methods are addressed. The final part of this chapter reviews and evaluates several applications of GMMs currently employed in commer- cial ventures.