The central premise of this wide-ranging book is that the current trajectory of human activity is not sustainable. Continued growth in income and population -- marks of success by some measure -- will not lead to an exhaustion of natural resources, as is sometimes feared. It will, however, lead to increased ecological stress, especially in the forms of climate change, a loss of biodiversity, and regional shortages of fresh water. Combined with persistent poverty and continued population growth -- particularly among idle young men -- this stress will lead to civil turmoil, transborder migration, and fragile or failed states vulnerable to terrorism and crime. For Sachs, these challenges are not occasion for despair about the future but rather lead him to call for vigorous cooperative action on a global scale -- requiring a markedly different approach to foreign policy by all countries, especially the United States. In his view, the challenges are serious but soluble, and at a modest cost relative to Americans' wealth; he offers numerous concrete suggestions for action.