The efficient use of energy, or energy efficiency, has been widely recognized as an ample and cost-efficient means to save energy and to reduce greenhouse gas emissions. Up to one-third of the worldwide energy demand in 2050 can be saved by energy efficiency measures. In this chapter, several important aspects of energy efficiency are addressed. After an introduction and definition of energy efficiency, historic development and state of the art and future trends of energy efficiency are presented in light of life cycle assessment and total cost of ownership considerations. Energy efficiency in the sector’s energy production, energy transmission and storage, transportation, industry, buildings, and appliances are reviewed. Concurrent measures such as recycling or novel materials are also discussed and touched upon. Energy conservation is covered in the final section of this chapter. References for deeper study are provided with an emphasis on guidelines on how to improve energy efficiency. Given the breadth of the subject, only exemplary coverage can be aimed for. The purpose of this chapter is to highlight the significance of energy efficiency and to provide cross-learnings from achievements in different sectors so that energy efficiency in the readers’ own facilities and installations can be assessed and improved with cost-effective means as a contribution to climate change mitigation, cost savings, and improved economic competitiveness.