A variety of organizations and research groups have undertaken analyses in the last four years to suggest how greenhouse gas emissions could be reduced in the range of 80 percent. The studies vary significantly in the mix of energy efficiency improvements, electrification, energy storage, demand-side management, transmission, renewable energy, nuclear power, and fossil energy with carbon capture and storage, and how they provide for fuel demand. The critical assumptions made by each study differ as well. As a whole, these studies indicate that the global decarbonization challenge is much larger than often reckoned. The level and pace of energy system transformation required to meet even 80 percent carbon reduction targets is substantially higher than historical experience, removing any particular technology from consideration is likely to limit the chance of success, and substantial investments in research and development will be required to ensure a range of feasible decarbonization options. This session explores the decarbonization studies with a focus on making the assumptions explicit, relating them to historical experience, and describing key implementation hurdles and solutions. Most importantly, the session identifies the elements common to all the studies, as these are indicative of robust strategies, and highlights the differences to illuminate the kinds of technical, economic, and policy choices we face in reaching the goal of radical emission reductions.