Abscisic acid (ABA) has been shown to regulate many physiological and developmental processes which are often mediated by the induction and suppression of gene expression. Herein, we review the progress made in the understanding of gene expression regulated by ABA, with the emphasis on cis- and trans-acting elements controlling gene expression. Promoter sequences containing an ACGT-core have been shown by several groups to be necessary for the ABA-induced gene expression. However, similar ACGT-core-containing sequences are also necessary for responses to a variety of environmental and physiological cues. To address the question what determines the response specificity, we have studied two barley ABA inducible genes and defined the modular nature of ABA response complex (ABRC), the promoter unit necessary and sufficient for ABA induction of gene expression. ABRCs of these genes consist of a 10-bp element with an ACGT-core (ACGT-box) and a coupling element (CE1 or CE3). These ABRCs function in both seed and vegetative tissues. Genetic analysis has led to the cloning of genes, such as maize Viviparous 1 (VP1), involved in the regulation of sensitivity of plants to ABA. In seeds, ABA induction of the ABRC containing CE3, but not the ABRC with CE1, is enhanced in the presence of the transcription regulator, encoded by the VP1 gene, indicating these two ABRCs are mediated by different ABA signal transduction pathways. Other potential signal transduction components mediating ABA signal transduction pathways are also discussed.