A powerful means to learn about gene functions in a developmental or physiological context in an organism is to isolate the corresponding mutants with altered phenotypes. Diverse mutagenic agents, including chemical and biological, have been widely employed, and each comes with its own advantages and inconveniences. For Arabidopsis thaliana, whose genome sequence is publicly available, the reliance of reverse genetics to understand the relevant roles of genes particularly those coding for proteins in growth and development is now a common practice. Identifying multiple alleles at each locus is important because they can potentially reveal epistatic relationship in a signaling pathway or components belonging to a common signaling complex by their synergistic or even allele-specific enhancement of the phenotypic severity. In this article, we describe mutagenesis by using ethyl methanesulfonate (EMS) and transfer (T)-DNA-mediated insertion or activation tagging as applied to the most widely used genetic plant model A. thaliana. Also, we demonstrate the utility of several genetic screening approaches to dissect adaptive responses to various abiotic stresses.