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 including: an ideal set of climate policy tools; the size of the economic and environmental costs of incomplete country or sector coverage of climate change mitigation policies;  how to concretely develop a global carbon market; the case for, and what can we reasonably expect from, R&D and technology support policies; and  the incentives for major emitting countries to join a climate change mitigation agreement.