Because of the importance of indole-3-acetic acid (IAA) in the growth and development of plants, extensive studies of the biosynthesis of IAA have been performed during the four decades since the discovery of IAA as a plant hormone. The pathway for the biosynthesis of IAA in plants remains, however, to be unelucidated, even though studies within the past decade have revealed unexpected aspects of such biosynthesis. By contrast, two pathways to IAA have been characterized in bacteria at the molecular level: the indole-3-acetamide (IAM) pathway (L-tryptophan-->IAM-->IAA); the indole-3-pyruvic acid pathway (L-tryptophan-->indole-3-pyruvic acid-->indole-3-acetaldehyde-->IAA) (Fig. 1). In both pathways, the details of the biosynthesis of IAA were clarified using IAA-overproducing bacteria. After a description of recent advances of the studies of the biosynthesis of IAA in plants, this review focuses on the excessive production of IAA in several organisms and its significance in the studies of the biosynthesis of IAA.