Auxin signaling is key to many plant growth and developmental processes from embryogenesis to senescence. Most, if not all, of these processes are initiated and/or mediated through auxin-regulated gene expression. Two types of transcription factor families are required for controlling expression of auxin response genes. One of these, the auxin response factor (ARF) family, functions by binding to auxin response elements (AuxREs) on promoters of auxin response genes, activating or repressing the auxin response genes, and recruiting a second family of transcription factors, the Aux/IAA repressors, that confer an auxin response to the genes. Recent advances have provided information on regulation of ARF gene expression, ARF roles in growth and developmental processes, and target genes regulated by ARFs.