Salinity often affects irrigated areas in arid and semi-arid regions of the world. The existence and accumulation of soluble salts in the soil layers limit the growth of crops essential for our food. Salt stress dramatically affects plant growth, plant development, as well as crop yield. Arabidopsis thaliana is the plant model that provides a comprehensive knowledge of plant development, genetics and physiology, and response to abiotic stresses such as salinity. The redundancy of genes due to duplication, even in the simple model genome of Arabidopsis, limits the value of knockout (KO) mutagenesis to provide complete information on gene function. 'Gain-of-function' mutants are an alternative genetic tool to identify gene functions for redundant genes, and those with small effect or that respond to an environmental condition. Transposon-mediated 'activation tagging' is an efficient genetic tool that can randomly generate 'gain-of-function' mutants for a large number of genes. In the method used here, the transposable element Enhancer-Inhibitor (En-I/dSpm) system of maize was modified to develop an activation tag (AT) mutant library in Arabidopsis. The mobile I-AT transposon contains a transcriptional enhancer, from the cauliflower mosaic virus (CaMV) 35S promoter, located close to the right border of the transposon. This I-AT element was mobilized to randomly insert into the plant genome by transposition from the T-DNA, and can give rise to mutants differing in the level of overexpression of the adjacent genes. Consequently, the gain-of-function dominant phenotypes generated are displayed by the I-AT plants due to enhanced expression of the gene(s) adjacent to the 35S enhancer. In this study, the I-AT library was used to screen for salt tolerance, identified by enhanced growth or biomass of the tagged mutants compared to the wild-type grown in saline conditions. A number of tagged salt tolerance candidate genes were identified flanking the I-AT insertion, and their tagged genes characterized for their role in salt tolerance.