To introduce this special issue of Social Science History, my essay sketches the historiographical context out of which grew Joachim Radkau's (2008) book Nature and Power: A Global History of the Environment (German edition 2002 ). It notes a few of the problems that Radkau and other historians face when undertaking a solo synthesis of research mainly at the local and regional levels. In addition to the issues of selecting themes, geographic extent, and temporal depth, questions of purpose have to be considered. Should global environmental history devote itself to producing a usable past for policies of the present and future? Should it, rather, take a prospective approach to reconstruct environmental thinking and practice as it was in a previous time, the better to identify unintended consequences and ambiguities in the historical past and to avoid presentism and anachronism while recognizing that history offers teachable moments? Radkau hews more to the second of these approaches than to the first, and his book provides the focus and a point of departure for the rest of the essays in the issue.