The scientific origins of ABA can be traced to several independent lines of investigation in the late 1940s (for review, see Addicott and Carns, 1983). In these studies, which in retrospect appear to have involved extracts that were rich in ABA, acidic inhibitors of coleoptile growth were detected that were also correlated with promoting activities on abscission and dormancy. It was only in the 1960s, however, that ABA was finally isolated and identified (for review, see Addicott and Carns, 1983). In 1963, Ohkuma and colleagues (Addicott and Carns, 1983) reported the crystallization of a substance that promoted abscission of young cotton fruits, which they called abscisin II. Shortly afterward, Wareing and colleagues purified from sycamore leaves a substance called dormin, which promoted bud dormancy (reviewed by Addicott and Carns, 1983). When dormin was chemically identified it was found to be identical to abscisin II. It was decided in 1967 to use ABA as the name, even though we now know that ethylene rather than ABA is actually the major factor causing organ abscission. Since the 1960s significant progress has been made in understanding how ABA controls diverse, essential physiological processes such as seed development and germination, as well as plant adaptation to abiotic environmental stress (for review, see Zeevaart and Creelman, 1988; Giraudat et al., 1994). Many of the actions of ABA, in both seeds and vegetative tissues, involve modifications of gene expression at the transcriptional level. Multiple ABAresponsive genes have been isolated, and their analysis has provided insights into the biological function of the encoded proteins, as well as into the nature of the cisand trans-acting factors involved in ABA responsiveness (for review, see Ingram and Bartels, 1996). A distinct type of ABA response is exemplified by the regulation of stomata. Stomatal aperture is defined by the turgor of the two surrounding guard cells. Guard cell volume is controlled osmotically mainly by large influxes (stomatal opening) or effluxes (stomatal closure) of K+ and anions. A variety of single-cell techniques have established that ABA causes rapid (within minutes) alterations in the activity of K+ and anionic channels in the plasma membrane of guard cells via pHand Ca2+sensitive signaling cascades (for review, see Blatt and Thiel, 1993; Ward et al., 1995). In recent years substantial progress toward understanding ABA action has come from combining classical genetics with modern methods for cloning the corresponding loci. In this Update we will illustrate how genetic analyses in model species such as Arabidopsis thaliana and maize (Zea mays) have shed new light on ABA biosynthesis, physiology, and signal transduction.