Abstract. It is now clear that drying of the soil does not always result in an early change in shoot water status. This may be because stomata close and leaf growth slows to reduce water loss. When this is the case, it is necessary to ask how the change in soil water status has been ‘sensed’by the shoot. The current view is that soil drying results in some type of chemical signalling between roots and shoots. The sensitivity of the response and experiments involving the manipulation of small parts of root systems suggest that the signalling involves more than a simple change in root activity in response to soil drying. In this paper, we consider the evidence for chemical signalling between roots and shoots and discuss the possible candidates for such signals. In some plants, root-sourced ABA can apparently influence shoot physiology and growth in the absence of any perturbation of shoot water relations. The ABA produced is quantitatively sufficient to account for the responses observed. Applied ABA can mimic many of the effects of soil drying on plants, including effects at the plasma membrane and on gene expression. Perhaps uniquely, ABA seems to be involved in signalling between different plant organs, and in signalling at the transmembrane and genome levels. We review the effects of ABA on leaf cells with a view to gaining some understanding of how soil drying may influence plant development.