Jeffrey Sachs, the director of the New York based Earth Institute, has established a formidable reputation as someone who thinks hard, and worries even harder, about the future of the planet. His latest book, Common Wealth, like its predecessor, The End of Poverty, reviews the major issues of international economic development in the early 21st century. But Common Wealth tries to go further than The End of Poverty, and raises the argument to a higher level of moral concern and practical difficulty. The defeat of poverty would be challenging in the best of circumstances, but global conditions in the early 21st century will be particularly hostile. Sachs identifies a conjunction of new problems, most of them arising from mankind’s pressure on the earth’s limited resources.