The phytohormone auxin is involved in almost all developmental processes in land plants. Most, if not all, of these processes are mediated by changes in gene expression. Auxin acts on gene expression through a short nuclear pathway that converges upon the activation of a family of DNA-binding transcription factors. These AUXIN RESPONSE FACTORS (ARFs) are thus the effector of auxin response and translate the chemical signal to the regulation of a defined set of genes. Given the limited number of dedicated components in auxin signaling, distinct properties among the ARF family likely contributes to the establishment of multiple unique auxin responses in plant development. In the 2 decades following the identification of the first ARF in Arabidopsis much has been learnt about how these transcription factors act, and how they generate unique auxin responses. Progress in genetics, biochemistry, genomics and structural biology have helped to develop mechanistic models for ARF action. However, despite intensive efforts, many central questions are yet to be addressed. In this review we highlight what has been learnt about ARF transcription factors, and identify outstanding questions and challenges for the near future.