Regional carbon balances, funded, for the Italian side, by the Italian Ministry of Environment in the context of a bilateral agreement to develop scientific collaborations in Global Change Research between Italy and USA signed in 2001. The two regions selected are Italy and Oregon-California; there are many similarities between these two regions (climate, vegetation, topography, population pressure, etc.) but, on other hand, there are also interesting contrasts in societal aspects like demography, land-use history and emissions. The main CarbIUS objectives are 1) the identification of spatial and temporal variability of carbon sources and sinks and the relative contribution of the different anthropogenic and biogenic components, 2) the impact of land use changes and human population dynamics on the carbon balance, 3) the quantification of the effects of climate and natural disturbances on the terrestrial carbon stocks and fluxes and 4) the application of new methodologies to investigate carbon metabolism at the plot, ecosystem and regional scale. In this paper will be presented the methodologies that we are using to assess the contribution of the different components to the full carbon budget, like carbon stocks and fluxes, disturbances (harvesting, wild forest fires and forest pathology), CH4 and NO2 fluxes and anthropogenic emissions. All these information will be input in a Data Assimilation System and the results will be validated using sub-regional airborne measurements of carbon fluxes.