After a short historical review, the basic principles for the experimental analysis of the mode of action of auxin are described. The biophysical basis of the auxin (IAA)-mediated growth response of excised coleoptile segments is then summarized. New evidence for the role of the outer epidermal wall in the control of coleoptile elongation is provided. A comparison of IAA-induced and fusicoccin (FC)-mediated growth reveals that the acid-growth hypothesis accounts for the FC-effect in the coleoptile. In contrast, IAA-induced growth is not mediated by acid secretion. The implications of this fact for the currently popular expansin-hypothesis of wall expansion are discussed. New results on the occurrence of IAA-induced osmiophilic particles at the outer epidermal wall are described. The evidence for the protein secretion theory of auxin-mediated cell-wall loosening is discussed and a new model of wall expansion is proposed.