The Cetacea (whales, dolphins and porpoises) currently include 86 species that inhabit the world's oceans, lakes and rivers. Some are cosmopolitan, occurring in marine waters from pole to pole, while others are restricted by ecology and population status to small ranges of a few thousand square kilometers or less. Some are exclusively marine, others are freshwater, and some are both.

The cetaceans arose deep in the artiodactyls (even-toed ungulates: cattle, sheep, antelopes, camels, pigs, etc.); their closest living relatives are the hippos. Formerly considered an order, they are now included in the Order Cetartiodactyla (cetaceans and their ungulate relatives). The taxonomic rank Cetacea should have (suborder, infraorder, or something else) is not yet agreed; more molecular phylogenetic research is needed.

Taxonomic coverage of the database is limited to the living species, although one of them, the baiji or Yangtze river dolphin is possibly now extinct. Coverage of taxa includes families, genera, subgenera, species and subspecies. Names covered include junior synonyms, unjustified emendations, suppressed names, misspellings and other non-operative names; this is to help deal with the older literature. The core of the World Cetacea Database (WCD) is derived from the World Register of Marine Species (WoRMS), which in turn is based on several taxonomic references (Wilson and Reeder's Mammal Species of the World; Rice's Marine Mammals of the World, Hershkovitz' Catalog of Living Whales, Perrin et al.'s Encyclopedia of Marine Mammals, and others). It will be updated as changes occur and new taxa are described.

Please inform the editor, William Perrin of any omissions, typos, or errors you encounter. The editor is also happy to review and discuss other issues, such as synonymies, controversial taxonomic assignments, or broader classification questions and to initiate changes if they are warranted. All inquiries and discussion will be evaluated promptly but critically, and if deemed reasonable included quickly into the database.