Descurainia is a genus in the Brassicaceae distributed throughout portions of the temperate Old and New World. The genus is most diverse in western North America and western South America, with a smaller center of distribution in the Canary Islands and three additional Old World species. Descurainia is well known for its taxonomic complexity, especially within New World species, on account of its numerous intergrading forms coupled with circumscriptions dependent upon inconsistent and overlapping characters. A molecular-based analysis of Descurainia was conducted using DNA sequences from nuclear ribosomal ITS and non-coding cpDNA regions. Descurainia and related genera form the monophyletic tribe Descurainieae, which is divided into two lineages: 1) Hornungia and Tropidocarpum and 2) Descurainia (including Hugueninia), Ianhedgea, and Robeschia. The genus is strongly-supported as monophyletic, and appears to be of Old World origin with recent diversification within the Canary Islands and the New World. Within the New World, three major well-supported lineages can be identified, with South American taxa distributed into four clades that correlate well with fruit dehiscence and orientation. A phylogeny recovered from combined ITS and cpDNA data is not well-resolved with respect to relationships between some major New World lineages, but suggests that multiple independent dispersals of Descurainia have taken place between North and South America. Substantial incongruence between ITS and cpDNA phylogenies, as well as the presence of mixed ITS sequences, point to a complex evolutionary history involving extensive gene flow and hybridization for North American Descurainia.