In this article the authors define sustainable development as an economic programme along which social well-being does not decline over time. It can be shown that the requirement is equivalent to the maintenance of a comprehensive measure of wealth, where an economy’s wealth is defined to be the social worth of its entire array of capital assets, including natural capital. Using data published by the World Bank on the world’s poorest regions, countries which would be regarded as having performed well if judged on the basis of such indices as GNP per head or the Human Development Index are found to have grown poorer, a few alarmingly so.