Plant hormones are naturally occurring organic molecules that regulate multiple aspects of plant growth and development at concentrations much lower than nutrients and other metabolites. At least five classes of plant hormones, including auxins, cytokinins, gibberellins (GAs), ethylene, and abscisic acid (ABA), have been characterized and studied in detail for many decades with respect to their occurrence and biosynthesis in plants, their molecular mechanisms of action, and the resulting alterations in cellular structure and function that lead to defined physiological responses to each hormone. In the past decade, a sixth class of plant hormones, brassino-steroids (BRs), has been added to the list of essential regulators of plant growth and development.