Working Group III of the Intergovernmental Panel on Climate Change has assessed mitigation options for the past 20 years. This paper reviews its assessments of bioenergy, geothermal, hydropower, ocean energy, solar, wind, nuclear, and carbon capture and storage (CCS), respectively, over that period. The assessments of hydropower, nuclear, and geothermal tend to focus on the social, economic, and environmental considerations that inhibit expansion of these mature technologies. The assessments of bioenergy, solar, and wind reflect their technological progress and market penetration. Assessments of ocean energy and CCS conclude they are likely to make only a modest contribution to mitigation efforts by 2030. The paper also reviews the assessments of policies to promote low carbon energy technologies.