Background: The largest terrestrial species in the order Carnivora are wide-ranging and rare because of their positions at the top of food webs. They are some of the world's most admired mammals and, ironically, some of the most imperiled. Most have experienced substantial population declines and range contractions throughout the world during the past two centuries. Because of the high metabolic demands that come with endothermy and large body size, these carnivores often require large prey and expansive habitats. These food requirements and wide-ranging behavior often bring them into confl ict with humans and livestock. This, in addition to human intolerance, renders them vulnerable to extinction. Large carnivores face enormous threats that have caused massive declines in their populations and geographic ranges, including habitat loss and degradation, persecution, utilization, and depletion of prey. We highlight how these threats can affect the conservation status and ecological roles of this planet's 31 largest carnivores. Advances: Based on empirical studies, trophic cascades have been documented for 7 of the 31 largest mammalian carnivores (not including pinnipeds). For each of these species (see fi gure), human actions have both caused declines and contributed to recovery, providing "natural experiments" for quantifying their effects on food-web and community structure. Large carnivores deliver economic and ecosystem services via direct and indirect pathways that help maintain mammal, avian, invertebrate, and herpetofauna abundance or richness. Further, they affect other ecosystem processes and conditions, such as scavenger subsidies, disease dynamics, carbon storage, stream morphology, and crop production. The maintenance or recovery of ecologically effective densities of large carnivores is an important tool for maintaining the structure and function of diverse ecosystems. Outlook: Current ecological knowledge indicates that large carnivores are necessary for the maintenance of biodiversity and ecosystem function. Human actions cannot fully replace the role of large carnivores. Additionally, the future of increasing human resource demands and changing climate will affect biodiversity and ecosystem resiliency. These facts, combined with the importance of resilient ecosystems, indicate that large carnivores and their habitats should be maintained and restored wherever possible. Preventing the extinction of these species and the loss of their irreplaceable ecological function and importance will require novel, bold, and deliberate actions. We propose a Global Large Carnivore Initiative to coordinate local, national, and international research, conservation, and policy.