Freshwater crayfi sh have long been used for studies in vision, neurobiology, molecular evolution, ecology and conservation. They are of ecological signifi cance as certain species play a role as keystone organisms in many habitats and invasive species in others (Crandall and Buhay 2008). Until somewhat recently, few studies had thoroughly examined the phylogeny of this group. Hobbs and Riek in the 1950s-1970s helped to organize the known species into two superfamilies, consisting of three families and 29 genera (Table 14.1, Hobbs 1974). This organization was based on morphological characters and geographical distribution. The monophyly of the superfamilies and all freshwater crayfi sh is supported by ribosomal and mitochondrial DNA evidence (Crandall et al. 2000) and morphological data (Rode and Babcock 2003, Karasawa et al. 2013). Specifi cally, the analysis by Karasawa et al. supported the monophyly of all crayfi sh in the infraorder Astacidea by the loss of marginal rostral spines. The monophyly of Astacoidea is supported by the development of complete diaresis in the telson and spiral element in the male pleopod 2 endopod. A synapomorphy that supports the Parastacoidea superfamily is the absence of the male pleopod 1 (Karasawa et al.