A powerful approach for determining the biological functions of genes in an organism is to produce mutants with altered phenotypes and physiological responses. Various approaches for mutagenesis involving chemical, irradiation, and insertional methods have been developed; each has advantages and disadvantages for the study of gene function. In this post-genomic era, the use of reverse genetic approaches to understanding the role of genes in growth and development has become widespread. With development of new techniques such as targeting induced local lesions in genomes (TILLING), ethyl methanesulfonate (EMS) mutagenesis can be used for both forward and reverse genetic studies. Generation of diverse mutant alleles in the same gene provides critical tools to understand the role of these genes in the function of the organism. Here we describe the general method of EMS mutagenesis for the molecular genetic model plant Arabidopsis thaliana .