Because of their expansiveness, productivity, and high proportion of total biomass allocated to belowground tissues, grazinglands might be expected to play an important role in the global carbon (C) cycle. Like forests and crop lands, grazinglands are thought to offer opportunities for sequestration of atmospheric C. Measurements made on central and western U.S. grazinglands by participants in the USDA-ARS Rangeland Flux Network are providing definitive documentation that most undisturbed grazinglands represent a sink for atmospheric C. Network results also showed that fire enhanced annual C fluxes into a tallgrass prairie but not in a sagebrush steppe. Grazing had little apparent effect on C fluxes in shortgrass steppe or northern mixed prairie; however, measurements of soil C content at these sites suggested that moderate grazing for decades led to increased soil C contents. A similar response to grazing was found in the Southeast. Network flux measurements and data from other studies indicate that variation in vegetation productivity due to environmental conditions or changes in management will likely result in associated changes in soil organic C content. However, C fluxes into grazinglands are positive in years with average or better growing conditions, and C appears to be accumulating in these ecosystems because gains made in average and better years exceed losses of C in years with limited rainfall or other adverse conditions. These positive C fluxes observed at sites in the USDA-ARS network suggest that U.S. grazinglands are currently storing amounts of C equal to recent estimates of the the "potential" of grazinglands to sequester atmospheric CO₂.