The freshwater crayfishes are a group of decapod crustaceans that have played a critical role in a diversity of biological studies, from physiology, to ecology, neurobiology, conservation, and evolution. Central to many of these fields of study is the dependence on a robust taxonomic framework for accurate communication relating to species diversity and associated attributes. Despite a huge body of taxonomic work since Linnaeus, there has never been a single, comprehensive taxonomic summary of all the species of crayfish of the world. There has also been an abundance of recent taxonomic work in terms of new species descriptions and taxonomic insights gained from a variety of phylogenetic studies. Here we gather diverse taxonomic and phylogenetic information into a single resource. We develop an updated classification system that includes all the crayfishes worldwide and taxonomic changes to better reflect the current phylogenetic knowledge of the group. We also include all the fossil crayfish taxa for a complete classification of extant and extinct crayfishes. Our classification results in two superfamilies (Astacoidea and Parastacoidea), five families, 38 genera, and 669 species (692 including distinct subspecies). We provide a checklist of all species and include validated taxonomic authorities, type localities, figure references, and synonyms. We also provide arguments for our revised classification. The updated and complete classification aims to provide a robust framework for future studies of the freshwater crayfishes of the world.