The atmospheric budget and distribution of acetone are investigated by using a priori estimates of sources and sinks to constrain a global three-dimensional atmospheric model simulation and then using atmospheric observations from 14 surface sites and 5 aircraft missions to improve these estimates through an inversion analysis. Observations over the South Pacific imply a large photochemical marine source of acetone, either from the ocean or from marine organic aerosol. Low concentrations of acetone measured at European sites in winter-spring and in the Arctic in summer suggest a large microbial ocean sink. The summer-to-fall decrease of concentrations observed in Europe argues against a large source from plant decay. Continental observations in the tropics and at northern midlatitudes in summer imply a large source from terrestrial vegetation. Observations in the Northern Hemisphere outside summer imply a large source from atmospheric oxidation of anthropogenic isoalkanes (propane, isobutane, isopentane). Model simulation of isoalkanes and comparison to observations yields best global emission estimates of 12 Tg C yr-1 for propane (including only 0.6 Tg C yr-1 from biomass burning), 3.6 Tg C yr-1 for isobutane, and 5.0 Tg C yr-1 for isopentane. Our best estimate of the global acetone source is 95 Tg yr-1. The mean tropospheric lifetime of acetone is estimated to be 15 days. Terrestrial vegetation and oceans are the principal sources of acetone in the tropopause region (0.1--0.7 ppbv) except in the extratropical Northern Hemisphere, where oxidation of isoalkanes is more important.