The evolutionary relationships among the major groups of crustaceans—and among crustaceans and other arthropods—have been hotly debated for many years, first based (mainly) on morphology alone and later based on inferences derived from molecular data as well. Different analytical approaches and data sets have sometimes yielded very different conclusions that have been difficult to reconcile. Disagreements persist not only about the relationships among groups, but also about the delineation of the groups themselves and their naming and appropriate taxonomic rank. Given the extraordinary diversity of the Crustacea, it is perhaps unsurprising that addressing these complex questions has been so challenging.