Brazil is the fifth largest country in the world and the first of the megadiversity countries, accounting for roughly 14% of the world's biota. It has the largest mammal diversity (more than 530 described species) with many yet to be discovered and cataloged. Very few sites have been adequately surveyed, and local lists are usually incomplete, which makes for knowledge gaps that hamper conservation and management initiatives and regional analyses. According to the Brazilian Institute for the Environment (IBAMA), 66 species are threatened, and the World Conservation Union lists 74. Primates, mostly Atlantic Forest endemics, are the most endangered group (40% of the threatened taxa). Carnivores and rodents are also notable members of the lists. Twenty-nine percent of listed species are marine, 18% occur in the Atlantic Forest; 13% in the Pampas, 12% in the Cerrado, 11% in the Pantanal, 7% in the Amazon, and 6% in the Caatinga. Human-induced habitat loss and fragmentation are major threats to land mammals in Brazil, and large- and medium-sized mammals are hunted. The major threat to small mammals is the scarcity of basic scientific knowledge-taxonomy, systematics, distribution, and natural history. Aquatic mammals are threatened by commercial hunting, accidental netting, long-line fisheries, chemical pollution, habitat degradation, tourism, and boat traffic. Mammal conservation has benefited from a number of recent initiatives by government and nongovernmental organizations, including legislation, nationwide initiatives to define priority areas for conservation, management plans for some threatened species, sustainable landscape planning, and new protected areas. Given the rate of habitat loss, a nationwide program of short- and long-term field surveys and increased support for natural history collections is of particularly urgency. Progress has been made toward conserving Brazilian mammals in recent years, but threats are growing fast, and conservation science must keep growing to provide the wherewithal to minimize and remove these threats.