Against the background of a projected doubling of world greenhouse gas emissions by mid-century, this book explores feasible ways to abate them at least cost. Through quantitative analysis, it addresses key climate policy issues: What would an ideal set of climate policy tools look like? How large are the economic and environmental costs of incomplete country or sector coverage of climate change mitigation policies? What are the pros and cons of policy tools to broaden that coverage, such as international sector-wide agreements or border-tax adjustments? What are the main challenges in incorporating a mechanism to reduce emissions from deforestation and forest degradation? How can we concretely develop a global carbon market? What is the case for, and what can we reasonably expect from, RandD and technology support policies? How great are the incentives for major emitting countries to join a climate change mitigation agreement, in terms of the costs and benefits (including the co-benefits from reduced local air pollution and improved energy security) of action? How can they be enhanced? How can international transfers of resources and technologies broaden support for action?