The review will focus on recent advances in our understanding of the action of auxin on plant cell elongation. Particular emphasis will be placed on the acid growth theory of auxin action and a comparison of auxin action on shoot vs. root tissues. Theories of the action of auxin will be evaluated in view of recent information on auxin dose/response relationships, auxin uptake kinetics, the time‐dependent adaptation of tissues to auxin, and the kinetics of plant cell responses to the hormone. Considerations of research on auxin binding and auxin polar transport will be included only as they apply to the central question of the action of the hormone on cell expansion.