The late Eocene-early Oligocene was a time of major changes in the composition and character of mammalian faunas. Broadly, the transition from archaic to modern eutherian families occurred at this time. This is a review of the main North American locality in which late Eocene mammalian faunas have been found and of the general faunal developments during the late Eocene. Major changes included the disappearance of several archaic carnivore and ungulate groups, decline of perissodactyls, the late dominance of protrogomorph rodents, beginning of a major radiation of artiodactyls, and progressive changes in several orders leading toward the appearance of more advanced families either in the late Eocene or early Oligocene. Correlation of the main upper Eocene mammal-bearing localities is discussed.