The phylogeny of freshwater crayfish is assessed using a morphological analysis of fossil and extant taxa. Based on external morphological characters, primarily characters of the carapace and appendages, the crayfish comprise a monophyletic group (Astacida). The sister taxon to the crayfish is the Chilenophoberidae, a clade of extinct marine lobsters. Synapomorphies of the freshwater crayfish include a distinctive carapace groove pattern and mobility of the last thoracic segment. Within the Astacida, three smaller clades, considered to be family-level groups, are identified and redefined. All parastacid genera except Gramastacus comprise a monophyletic clade. Species in the Cambaridae are characterized by an apomorphous annulus ventralis in the female, and hooks on the ichiopodites on the third pereiopod in the male. Species in the Astacidae are characterized by randomly arranged tubercules on the medial portion of dactyl. The monophyly of the Astacoidea (Cambaridae plus Astacidae) is also supported. The familial affinity of Cambaroides is questioned, and this genus is tentatively reassigned from the Cambaridae to the Astacidae.