Sustainable national income, (SNI) is an indicator for environmental sustainability, which gives an estimate of the production level at which - with the technology in the year of calculation - environmental functions remain available ‘for ever’. Overview The national income of a country is an estimate of the yearly production of goods and services. The loss of possible uses of the non-human made physical surroundings, named environmental functions, on which humanity is dependent in all its doings remains outside the estimate. Also the present and future production is dependent on these environmental functions. The sustainable national income (SNI) in a given year is an estimate of the production level at which - with the technology in the year of calculation - environmental functions remain available ‘for ever’. The sustainable national income (SNI) in a certain year is defined as: the maximum attainable production level whereby, with the available technology in the year of calculation, vital environmental functions remain available ‘for ever’. The production level in the same year that is registered in the standard national income (NI) does not meet this condition. Environmental functions and their preservation after all fall outside the NI. The NI is therefore always higher than the SNI. The difference gives information about the distance between the present production level and the production level in a sustainable situation. If the distance decreases then we are on the road to environmental sustainability, the part of the production that is based on unsustainable use of the environment decreases. If the distance increases then we are drifting further away from sustainability. History The original concept of ´sustainability´ refers to an equilibrium relation between human activities and their physical surroundings and has a long tradition going back to the nineteenth century. The sustainable national income is based on a definition of sustainability in conformity with this. The original concept of sustainability is introduced in the international discussion by 1980 publication of “The World Conservation Strategy” of 1980 Since the publication of “Our Common Future” in 1987 one has started to include in sustainability besides a sustainable use of the physical surroundings also elements that conflict with this, such as the growth of production as measured in the national income and some social measures. By taking together environmental conservation and herewith conflicting goals in one and the same sustainability indicator the development of the state of the environment is being obscured.