Auxin is a critical plant hormone involved in various developmental processes. This review discusses the complex mechanisms plants use to regulate auxin levels, its movement through the plant, signaling pathways, and interactions with other phytohormones. Indole-3-acetic acid (IAA) is the primary auxin in most plants, synthesized from tryptophan through both Trp-dependent and Trp-independent pathways. Plants can also obtain IAA by β-oxidation of indole-3-butyric acid (IBA) or by hydrolyzing IAA conjugates. Auxin signaling is mediated by an SCF TIR1 E3 ubiquitin ligase complex, which degrades Aux/IAA repressors in response to IAA, altering gene expression. Auxin interaction with other hormones adds complexity to understanding auxin responses. Despite over six decades of research, many aspects of auxin metabolism, transport, and signaling are well established, but many fundamental questions remain unresolved. Auxin activity is not limited to IAA; other compounds like 2,4-D, IBA, and NAA also exhibit auxin-like effects. Auxin biosynthesis occurs through various pathways, including the indole-3-pyruvic acid (IPA) pathway, the indole-3-acetamide (IAM) pathway, the tryptamine pathway, and the indole-3-acetaldoxime (IAOx) pathway. These pathways contribute to the regulation of IAA production, but the exact roles of each pathway are still under investigation. IAA can be stored in the form of conjugates, such as IAA–amino acid and IAA–sugar conjugates, which can be hydrolyzed to release free IAA. IBA, another endogenous auxin, is converted to IAA in peroxisomes, suggesting a link between IAA and IBA metabolism. Auxin conjugates are also involved in IAA inactivation, with pathways that counteract the inputs to the IAA pool. IAA conjugates can be further oxidized or conjugated to nonhydrolysable forms, contributing to IAA inactivation. The study of auxin conjugates and their metabolism is crucial for understanding plant growth and development.Auxin is a critical plant hormone involved in various developmental processes. This review discusses the complex mechanisms plants use to regulate auxin levels, its movement through the plant, signaling pathways, and interactions with other phytohormones. Indole-3-acetic acid (IAA) is the primary auxin in most plants, synthesized from tryptophan through both Trp-dependent and Trp-independent pathways. Plants can also obtain IAA by β-oxidation of indole-3-butyric acid (IBA) or by hydrolyzing IAA conjugates. Auxin signaling is mediated by an SCF TIR1 E3 ubiquitin ligase complex, which degrades Aux/IAA repressors in response to IAA, altering gene expression. Auxin interaction with other hormones adds complexity to understanding auxin responses. Despite over six decades of research, many aspects of auxin metabolism, transport, and signaling are well established, but many fundamental questions remain unresolved. Auxin activity is not limited to IAA; other compounds like 2,4-D, IBA, and NAA also exhibit auxin-like effects. Auxin biosynthesis occurs through various pathways, including the indole-3-pyruvic acid (IPA) pathway, the indole-3-acetamide (IAM) pathway, the tryptamine pathway, and the indole-3-acetaldoxime (IAOx) pathway. These pathways contribute to the regulation of IAA production, but the exact roles of each pathway are still under investigation. IAA can be stored in the form of conjugates, such as IAA–amino acid and IAA–sugar conjugates, which can be hydrolyzed to release free IAA. IBA, another endogenous auxin, is converted to IAA in peroxisomes, suggesting a link between IAA and IBA metabolism. Auxin conjugates are also involved in IAA inactivation, with pathways that counteract the inputs to the IAA pool. IAA conjugates can be further oxidized or conjugated to nonhydrolysable forms, contributing to IAA inactivation. The study of auxin conjugates and their metabolism is crucial for understanding plant growth and development.