Synthetic biology leverages advances in computational biology, molecular biology, protein engineering, and systems biology to design, synthesize, and assemble genetic elements for manipulating cell phenotypes. This emerging field is founded on a vast amount of gene sequence data available in public databases and our ability to rapidly and inexpensively synthesize DNA fragments of sufficient length to encode full-length genes, enzymes, metabolic pathways, and even entire genomes. Several thousand genetic elements encoding enzymes, reporters, repressors, activators, promoters, terminators, ribosome binding sites, signaling devices, and measurement systems are now available for engineering microbes. In addition to facilitating rational design, these new tools allow us to create and harness genetic diversity in combinatorial approaches to rapidly optimize metabolic pathways. As such, synthetic biology holds great promise for accelerating the development of microbial systems for the production of renewable fuels and chemicals.