The atmospheric budget and distribution of acetone are investigated by using best a priori estimates of sources and sinks to constrain a global 3-D 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, either from the ocean or from marine organic aerosol. Low concentrations 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 including 33 Tg yr-1 from terrestrial vegetation, 27 Tg yr-1 from the oceans, 21 Tg yr-1 from oxidation of isoalkanes, 6 Tg yr-1 from oxidation of monoterpenes, 5 Tg yr-1 from biomass burning, 2 Tg yr-1 from plant decay, 1 Tg yr-1 from atmospheric oxidation of methylbutenol, and 1 Tg yr-1 from anthropogenic emission. The tropospheric lifetime of acetone is estimated to be 15 days and is determined by losses from photolysis (45% of the global sink), oxidation by OH (30%), uptake by the ocean (15%), and deposition to land (10%). Terrestrial vegetation and oceans dominate the source of acetone in the tropopause region (0.1-0.7 ppbv) except in the extratropical northern hemisphere where oxidation of isoalkanes is more important. Better information on ocean-atmosphere and biosphere-atmosphere exchange is needed to improve our understanding of the acetone budget.