This paper, which was commissioned by the InterAcademy Council, discusses the importance and benefits of considering energy end-use efficiency when making energy economic decisions. Energy end-use efficiency is the concept of providing more desired services per unit of energy consumed. According to Amory Lovins, increasing energy end-use efficiency is generally the largest, least expensive, most benign and most neglected way to provide energy services. Making engineering decisions based on energy end-use efficiency allows systems to gain more work from energy through smarter technologies and find energy efficiency along energy conversion chains. According to this paper, there are multiple benefits to adopting energy end-use efficiency: improved services (more comfortable buildings with more attractive lighting), improved position in global fuel markets, and integrating efficiency with supply. The paper then compares the engineering and economic perspectives on energy efficiency as well as the relationship between diminishing and expanding returns to investments in energy efficiency.