In the wake of the global financial crisis, Keynesianism has had something of a revival. In practice, governments have turned to Keynesian policy measures to avert economic collapse. In the theoretical area, mainstream economists have started to give grudging attention to Keynesian perspectives previously dismissed in favor of New Classical theories. This theoretical and practical shift is taking place at the same time that environmental issues, in particular global climate change, are compelling attention to alternative development paths. Significant potential now exists for “Green Keynesianism” -- combining Keynesian fiscal policies with environmental goals. But there are also tensions between the two perspectives of Keynesianism and ecological economics. Traditional Keynesianism is growth-oriented, while ecological economics stresses limits to growth. Expansionary policies needed to deal with recession may be in conflict with goals of reducing resource and energy use and carbon emissions. In addition, long-term deficit and debt problems pose a threat to implementation of expansionary fiscal policies. This paper explores the possibilities for Green Keynesianism in theory and practice, and suggests that these apparent contradictions can be resolved, and that Green Keynesian policies offer a solution to both economic stagnation and global environmental threats.