© 2013 The Johns Hopkins University Press. All rights reserved. Metabolism can be thought of as having primary and secondary roles. Primary metabolism is what we generally regard as biochemistry. It is responsible for fundamental, life-sustaining pro cesses such as protein synthesis, transcription, and cell signaling cascades. Primary metabolites include amino acids, nucleotides, carbohydrates and lipids, and polymers thereof, but also include a large number of small molecule cofactors. They are ubiquitous in living organisms and function in similar fashion across phyla. Secondary metabolism does not necessarily sustain physiological function but nonetheless can have a role in supporting life. Building blocks of secondary metabolites are isoprene, acetate, and amino acids, resulting in, respectively, terpenes, polyketides, and small peptides and alkaloids. However, some secondary metabolites are modified primary metabolite monomers, which include oxidation or reduction products or esterified or glycosylated products. Regardless of biosynthetic origin, what distinguishes secondary metabolites are their functions, which include defense and communication, and their phylogenetic distribution, which is often species specific. The term natural products is often used by chemists to describe secondary metabolites, reflecting the origin in nature of this group of small molecules.