Energy efficiency refers to the ability of a system to decrease its energy consumption while providing the same level of service, and it is related with the technical performance of energy conversion and energy-consuming devices. Energy efficiency helps the consumers to decrease their energy consumption and thus pay less bills. This chapter provides the definition of energy efficiency and it clarifies the difference between energy efficiency and energy conservation. It also presents examples for energy efficiency applications. Further, the chapter discusses the obstacles for deployment of energy efficiency and provides suggestions on how to overcome those obstacles.