Among the many advances in our understanding of angiosperm relationships in recent decades due to the advent of DNA sequence data is the confirmation that all plants (apart from Drypetes) that produce mustard oil precursors are related to each other and should be treated as one order, Brassicales. Due to the lack of obvious shared morphological characters, this is one of the more unexpected of these advances. Here we give the background to this development and introduce the families in Brassicales, including Tropaeolaceae, the subject of this issue.