Extending conventional national product measures, green national accounting provides better indicators of economic welfare and of the sustainability of welfare levels. The main theoretical result shows that in an undistorted economy net national product is proportional to welfare, provided some rather stringent conditions are met. With appropriately used shadow prices, the welfare effects of externalities and world market changes can be accounted for and sustainable income – the hypothetical level of consumption that can be sustained into the future – can be calculated. Practical approaches have been proposed to adjust conventional national income figures roughly in the spirit of the theoretical results.