Abscisic acid (ABA) plays important roles in plant responses to abiotic and biotic stresses as well as in the regulation of seed development and germination. Most often physiological responses mediated by ABA are well correlated with hormone contents, which are determined by the balance between the rate of biosynthesis and catabolism. ABA derives from cleavage of carotenoid precursors and many of the enzymes involved in the ABA pathway (biosynthesis and catabolism) have been identified. Although enzymatic reactions involved in the production of cis-isomers of violaxanthin and neoxanthin from all-trans-violaxanthin are yet to be clarified, it is now evident that the reactions catalyzed by nine-cis-epoxycarotenoid dioxygenase (NCED) and ABA 8'-hydroxlase encoded by CYP707As are rate-limiting steps for the biosynthesis and catabolism, respectively. Several regulatory factors for ABA metabolism have also been reported, which contribute to the understanding of how plants respond to environmental changes during plant growth, seed development and germination at molecular levels. Furthermore, recent identification of several types of transporters that mediate influx or efflux of ABA, namely ATP-binding cassette (ABC) type, NITRATE TRANSPORTER 1/PEPTIDE TRANSPORTER FAMILY (NPF) and Detoxification Efflux Carrier (DTX)/Multidrug and Toxic Compound Extrusion (MATE) proteins, provided us with new insights into the spatial regulation of ABA concentrations.