Alligatoroidea, a stem-based group including living alligators and caimans and all taxa closer to them than to Crocodylus or Gavialis, includes seven living species and a large diversity of extinct taxa extending back to the Campanian (Late Cretaceous). Parsimony analysis of 164 discrete morphological characters supports several previous hypotheses based on morphology and molecules: Diplocynodon, from the European Tertiary, is a monophyletic alligatoroid lineage; Brachychampsa and Stangerochampsa are derived alligatoroid taxa but not demonstrably within the crown-group Alligatoridae; and, within the crown-group, caimans form a robust clade. However, this study indicates the paraphyly of "Allognathosuchus;" the alligatoroid affinities of Leidyosuchus canadensis but not of most other "Leidyosuchus;" the very distant relationship between Hylaeochampsa and any extant crocodylian lineage, including Alligatoroidea; and the fact that Diplocynodon must extend at least as far as the Campanian, even though the oldest well-known Diplocynodon are from the Eocene. This work supports a close relationship between Diplocynodon and Baryphracta deponiae from the Eocene of Germany; monophyly of Alligator and the caiman assemblage as currently applied in the literature; the caiman affinities of the bizarre nettosuchids and Purussaurus; and a closer relationship between Caiman latirostris and Melanosuchus niger than between C. latirostris and other Caiman. Phylogenetic taxonomic principles have been applied to Crocodylia by several workers, and this study expands upon this with a discussion of phylogeny-based name definitions for Crocodylia and nested subgroups. Leidyosuchus is the basalmost alligatoroid, but within its sister taxon we can recognize a pair of stem-based groups-Diplocynodontinae (Diplocynodon ratelii and all taxa closer to it than to Alligator mississippiensis) and Globidonta (Alligator mississippiensis and all taxa closer to it than to Diplocynodon ratelii). The crown-group Alligatoridae (last common ancestor of Alligator, Caiman, Melanosuchus, and Paleosuchus and all of its descendents) can likewise be split into a pair of stem-based daughter lineages: Alligatorinae (Alligator and taxa closer to it than to Caiman) and Caimaninae (Caiman and taxa closer to it than to Alligator). Salt intolerance has been used by previous authors to force alligatorid dispersal along land bridges. Despite the problem of inappropriate climate, a land crossing across Beringia during the Late Tertiary best explains the current existence of an Asiatic alligatorid (Alligator sinensis), but based on known fossil occurrences, caimans and alligators diverged from each other in North America sometime during the Late Cretaceous and spread to South America by the Paleocene, when North and South America were separated. A single dispersal event at or near the Cretaceous-Tertiary Boundary is sufficient to explain the presence of caimans in South America, with a reinvasion from South America explaining the presence of a nettosuchid in the North American Eocene (Orthogenysuchus).