Auxin response factors (ARFs), transcription factors (TFs), and their Aux/IAA (IAA) repressors are central components of the auxin signalling pathway. They interact as homo- and heteromultimers. The structure of their interacting domains revealed a PB1 fold mediating electrostatic interactions through positive and negative faces. Detailed structural analysis revealed additional hydrophobic and polar determinants and started unveiling an ARF/IAA interaction code. Structural progress also shed new light on the DNA binding mode of ARFs showing how they dimerize to bind repeated DNA elements. Here, we discuss the in vitro and in vivo significance of these structural properties for the ARF family of TFs and identify some critical missing information on how specificity might be achieved in the auxin signalling pathway.