of species diversity, and allow natural systems to regenerate. Many other commentators consider this a conservative estimate. The basic problem behind this consumption is not primarily inefficiency, but rather open-ended economic growth. As Anders Hayden (1999, p.26) has pointed out, without a commitment to reduce our demands on the Earth, continuing growth of the economy can offset even substantial efficiency gains. He cites advocates of Factor Four efficiency policies who have pointed out that with a growth rate of 5 percent a year all the gains for a factor-four [i.e. four-fold] efficiency revolution would be devoured in less than 30 years. At a more moderate 3 percent rate of economic growth, the efficiency revolution would be entirely undone in a mere 47 years. William Rees (1995) has even pointed to the rebound effect of efficiency gains which can stimulate even more consumption if the system is not consciously concerned with abating overall resource use. It is no exaggeration to say that this profligate growth and consumption is the single most important source of all our environmental problems and many of our social ones. Most of our pressing environmental problems are intimately connected with our use and misuse of materials. The depletion of the ozone layer, climate change, disruption of the global nitrogen cycle, the narrowing of species diversity, the destruction of the fisheries and the many forms of toxic pollutionare symptoms of either overuse of natures materials, or insensitive means of extracting, processing and using them. Eco-footprint is an environmental indicator that measures an economys (or a communitys or an individuals) overall environmental impact in terms of the territory needed to support their consumption. Materials use strongly influences the size of a populations footprint: in the U.S. case, materials are conservatively estimated to account