An updated classification of the Crustacea down to the level of family is provided. The classification is based loosely on that given by Bowman and Abele (1982) and includes all new families and higher level taxa described since that time. In addition, in several crustacean groupings, new arrangements and assignments have been incorporated, based usually on phylogenetic information that has accrued or that has become more widely accepted since 1982. Among the more salient changes, some of which are more controversial than others, are the recognition of the former phylum Pentastomida as a group of maxillopod crustaceans based on additional spermatological and molecular evidence, the inclusion of the parasitic Tantulocarida also among the maxillopods, the treatment of the Branchiopoda as the most primitive extant group of crustaceans, and the recognition of Guinot’s (1977, 1978) division of the higher (eubrachyuran) crabs into two ‘‘grades’’ based primarily on placement of the genital aperture. The revised classification includes 849 extant families in 42 orders and 6 classes; this is an increase of nearly 200 families since the Bowman and Abele classification. More than 90 specialists in the field were consulted and asked to contribute to the update. Some workers are not in agreement with our final arrangement. In particular, there are questions or dissenting opinions over our choice of which taxa to recognize, which authorities and dates to credit for various taxa, and especially over the arrangements among and/or within the higher taxa. As an aid to future workers in crustacean classification and phylogeny, comments and dissenting opinions of some of these workers are appended to highlight areas of uncertainty or controversy. Also appended are a list of the specialists who were given the opportunity to respond (Appendix II) and a list of printed and World Wide Web resources that contain information on crustaceans (Appendix III). The new classification is in part a result of one such site, the Crustacean Biodiversity Survey.