Efficient use of energy is in all countries the most important, economical, prompt, underused, overlooked, and misunderstood way to provide future energy services. It is rapidly becoming even larger, faster, and cheaper as technologies, delivery methods, and integrative design improve. Whole-system design can indeed make very large energy savings cost less than small ones. But capturing energy efficiency's remarkable potential requires careful terminology, prioritization, attention to engineering details and to market failures, and willingness to accept measured physical realities even if they conflict with economic theories. If well done, such energy efficiency can displace costly and disagreeable energy supplies, enhance security and prosperity, speed global development, and protect Earth's climate — not at cost but at a profit. This article appeared in the Encyclopedia of Energy, Volume 2. pages 383–401 (23 July 2004).