Dramatic advances in our understanding of auxin signal-response pathways have been made in recent years. Much of this new knowledge has come through the study of mutants in Arabidopsis thaliana. Mutations have been identified in a wide variety of auxin-response components, including auxin transporters, protein kinases and phosphatases, components of a ubiquitin-proteosome pathway, and transcriptional regulators. This review focuses on mutations that affect auxin-modulated transcription factors, in particular those in the Aux/IAA and AUXIN RESPONSE FACTOR (ARF) genes. Mutants in members of these related gene families exhibit phenotypes that indicate both unique localized functions, as well as overlapping redundant functions, throughout plant development - from embryogenesis to flowering. Effects of specific mutations on Aux/IAA and ARF protein functions at the biochemical and physiological levels will be discussed. We will also discuss potential mechanisms for interactions between auxin and light response pathways that are suggested by these mutants.