Mutants have been responsible for most of the recent achievements in plant hormone biology. Their importance cannot be overemphasized. The possession of lines that differ by only a single gene provides a powerful tool for following biosynthetic pathways, and for establishing the causal links between hormone and response that have been so hard to achieve by physiological and biochemical approaches. Moreover, the ability to clone a gene defined only by mutation has led to the isolation of many of the first genes known to be involved in hormone biosynthesis or signaling. The combination of mutants and cloned genes has opened the way for reverse genetic approaches that offer tremendous potential for the future. This chapter deals with some general considerations for the sucessful isolation of mutants and provides a mutagenesis protocol for Arabidopsis.