The name ‘‘abscisic acid’’ (ABA) was proposed in 1968 for the sesquiterpenoid hormone previously known as ‘‘dormin’’ or ‘‘abscisin II,’’ which had been isolated from several higher plants (Addicott and others 1968). In early studies its functions appeared mostly negative inasmuch as ABA treatments reduced growth and inhibited germination. However, it became clear that ABA was essential for normal growth and development, and we now know that ABA produces a huge range of both shortterm and long-term changes in cellular metabolism and processes. It has been estimated that 8–10% of Arabidopsis genes are ABA-responsive (Finkelstein and others 2002). As analytical methods improved, ABA was detected in algae, fungi, and liverworts in addition to higher plants (Hartung and Gimmler 1994). In this special issue, our progress toward understanding how ABA functions in higher plants is presented via reviews of selected topics. For many years the biosynthetic origin of ABA in higher plants was obscure. Although fungal pathways to ABA via farnesyl diphosphate were identified, these pathways could not be convincingly demonstrated in higher plants. Traditional tracer experiments to identify pathway intermediates were plagued by low incorporation rates. It became apparent that the biosynthetic route in fungi was quite different from that in higher plants. It is now accepted that ABA is produced by an ‘‘indirect’’