Four conclusions emerge from this review. First, the pace of energy system transformation required to meet deep carbon reduction targets is substantially higher, and in some cases orders of magnitude higher, than historical experience. At a minimum, achieving this pace of transition will require significantly more “technology push” and “demand pull” efforts and policies. Second, given the scale of the challenge and the substantial social, technical, and economic hurdles faced by all available low-carbon energy technologies, removing any particular technology from consideration is likely to limit the chance of success. Third, given the constraints associated with scale up of low carbon technology at the pace described in the scenarios, substantial investments in research, development, demonstration, and deployment (RDD&D) are required to expand and improve our range of feasible decarbonization options. Finally, future studies of decarbonization pathways should explicitly describe their assumptions, relate them to historical experience, and describe key implementation hurdles and solutions; otherwise, such studies are little more than arithmetic exercises describing hypothetical futures.