Abstract: The roles of immigration and endemic radiation of survivors of the extinctions marking the Cretaceous/Tertiary (K/T) boundary in evolution of the earliest Paleocene (Puercan) mammalian fauna of the North American Western Interior have been debated. Continued field and laboratory research has significantly expanded and refined knowledge of the compositions of North American latest Cretaceous (Lancian North American Land Mammal Age) and Puercan 1 (Pu1) local faunas. It has also revealed limitations in biochronological correlations of these local faunas. In general, Pu1 local faunas of the northern Western Interior reflect the extinction or extirpation of lineages of Lancian marsupials and consist primarily of multituberculate and eutherian mammals. The majority of Pu1 mammals were immigrants, many dispersing into the area soon after the K/T boundary. Testing the hypothesis of Asian origins of the immigrants is hampered by lack of latest Cretaceous or earliest Paleocene mammalian local faunas particularly from high North American paleolatitudes and Pacific coastal areas of Asia. Another working hypothesis, that most, if not all, of the Pu1 immigrant mammals entering the northern Western Interior could have dispersed from other areas of the North American continent has increasing support and has yet to be falsified.