The phytohormone, abscisic acid (ABA) is found in all photosynthetic organisms. The amount of ABA present is determined by the dynamic balance between biosynthesis and degradation: these two processes are influenced by development, environmental factors such as light and water stress, and other growth regulators. ABA is synthesized from a C40 carotenoid precursor and the first enzyme committed specifically to ABA synthesis is a plastid- localized 9-cis-epoxycarotenoid dioxygenase, which cleaves an epoxycarotenoid precursor to form xanthoxin. Subsequently, xanthoxin is converted to ABA by two cytosolic enzymes via abscisic aldehyde, but there appears to be at least one minor alternative pathway. The major catabolic route leads to 8'-hydroxy ABA and phaseic acid formation, catalyzed by the cytochrome P450 enzyme ABA 8'-hydroxylase. In addition, there are alternate catabolic pathways via conjugation, 4'-reduction and 7'-hydroxylation. As a consequence of recent developments, the mechanism by which the concentration of hormonally active ABA is controlled at the cellular, tissue and whole plant level can now be analyzed in detail.