Environmental or 'ecological' footprints have been widely used as aggregate indicators of the human appropriation of natural capital with prevailing technology. They represent a partial measure of a community's pathway towards sustainable development. Footprints vary between countries at different stages of economic development and varying geographic characteristics. Dimensional analysis techniques from engineering and the thermal sciences have been employed to determine the influence of a wide range of parameters on per capita national footprints, including per capita national income, population density, pollutant emission intensity, local climate, soil productivity, and technology. One hundred and nine countries made up the final database (based on 2003 international statistical data sets). Per capita national environmental footprints are found to be strongly dependent on per capita national income, and only weakly on population density. The implications of the findings are illustrated by reference to the situation in the G8 + 5 nations. Variations about the resulting power-law correlation suggest the extent to which individual nations are frugal or profligate in terms of their resource use and environmental impacts. The ecological debt owed by the industrialized countries of the North to the developing nations of the populous South is highlighted.