Energy Efficiency 2018, the sixth edition in the IEA Market Report Series, is the global tracker of trends and indicators in energy efficiency and an invaluable resource for energy efficiency policy makers and market actors. This year’s report provides a special feature in the form of a new World Energy Outlook Efficient World Scenario, which answers the question: What would happen if policy makers realised all the economically viable potential for energy efficiency that is available with existing technologies? This Efficient World Scenario and historic data are used to examine the following questions.