Two methods are used to estimate CO2 degassing from the Yellowstone magmatic-hydrothermal system. The amount of magmatic CO2 released as basaltic magma emplaces from the mantle into the crust beneath the Yellowstone caldera is calculated and compared to CO2 fluxes measured in three different types of hydrothermal regions within Yellowstone. Comparison of modeled estimates with surface measurements suggests that 3.7 ¿ 1.3 ¿ 1011 mol y-1 (45 ¿ 16 kt d-1) of CO2 are released from Yellowstone due to diffuse degassing. Flux measurements suggest that the diffuse flux in acid-sulfate regions is significant in total calculations (>96% of the total), whereas the diffuse flux in neutral-chloride and travertine-precipitating areas is not significant. Analyses of carbon and helium isotopes suggest that ~50% of the CO2 emitted is derived from sedimentary sources at locations outside the caldera, whereas locations inside the caldera likely have sedimentary contributions <30%. In addition to release of CO2 with emplacement, magma crystallization in the subsurface is thought to contribute significantly to the CO2 emissions at the surface. The contribution of CO2 from Yellowstone to global volcanic CO2 emissions (~6--7 ¿ 1012 mol y-1) is comparable to the CO2 contribution from other large volcanic systems like Popocatepetl, Mexico and the combined contribution from the Hawaii hot spot. Likewise, the amount of CO2 emitted per land area from Yellowstone (on average 108 mol CO2 km-2 y-1) is comparable to other large volcanic and hydrothermal systems worldwide.