Cloud Computing is growing its customer base due to its pay per use model of leased computational resources as well as software services. Along with the rapid growth of cloud computing adoption, the energy consumed by cloud computing infrastructure is growing. There is an urgent need for the research on the energy efficiency of cloud computing infrastructure. We reviewed the key technologies and techniques which will support and enhance the energy efficiency of cloud computing infrastructure and makes cloud a sustainable model. Virtualization, elasticity, and energy-efficiency are three important attributes of cloud and its infrastructure; we studied the interdependencies of these techniques and addressed few questions related to their interdependencies. © 2017 IEEE.