The capacity to produce the phytohormone indole-3-acetic acid (IAA) is widespread among bacteria that inhabit diverse environments such as soils, fresh and marine waters, and plant and animal hosts. Three major pathways for bacterial IAA synthesis have been characterized that remove the amino and carboxyl groups from the α-carbon of tryptophan via the intermediates indolepyruvate, indoleacetamide, or indoleacetonitrile; the oxidized end product IAA is typically secreted. The enzymes in these pathways often catabolize a broad range of substrates including aromatic amino acids and in some cases the branched chain amino acids. Moreover, expression of some of the genes encoding key IAA biosynthetic enzymes is induced by all three aromatic amino acids. The broad distribution and substrate specificity of the enzymes suggests a role for these pathways beyond plant-microbe interactions in which bacterial IAA has been best studied.