Energy efficiency plays and will continue to play an important role in the world to save energy and mitigate greenhouse gas (GHG) emissions. However, little is known on how much additional capital should be invested to ensure using energy efficiently as it should be, and very little is known which sub-areas, technologies, and countries shall achieve maximum greenhouse gas emissions mitigation per dollar of investment in energy efficiency worldwide. Analyzing completed and slowly moving energy efficiency projects by the Global Environment Facility during 1991-2010, Closing the Gap: GEF Experiences in Global Energy Efficiency evaluates impacts of multi-billion-dollar investments in the world energy efficiency. It covers the following areas: 1. Reviewing the world energy efficiency investment and disclosing the global energy efficiency gap and market barriers that cause the gap; 2. Leveraging private funds with public funds and other resources in energy efficiency investments; using these funds in tangible and intangible asset investments; 3. Investment effectiveness in dollars per metric ton of CO2 emissions mitigation in 10 energy efficiency sub-areas; 4. Major barriers causing failure and abandonments in energy efficiency investments; 5. Quantification of direct and indirect CO2 emissions mitigations inside and outside a project boundary; and 6. Classification and estimation of CO2 emissions mitigations from tangible and intangible asset investments. Closing the Gap: GEF Experiences in Global Energy Efficiency can serve as a handbook for policymakers, project investors and managers, and project implementation practitioners in need of benchmarks in energy efficiency project investments for decision-making. It can also be used by students, researchers and other professionals in universities and research institutions in methodology development for evaluating energy efficiency projects and programs.