This chapter starts with a high-level characterization of data center energy before moving to a discussion of power, energy, and sustainability in warehouse-scale computers (WSCs). We discuss the economic and environmental importance of energy efficiency, and define metrics like PUE (Power Usage Effectiveness) and SPUE (Server PUE) that help assess and improve data center and server infrastructure efficiency. We also explore the concept of energy-proportional computing and techniques for enhancing the energy efficiency of computing resources. We then discuss data center power provisioning strategies, including oversubscription and energy storage, and conclude with a discussion of environmental sustainability and a high-level characterization of data center energy usage of the internet and cryptocurrency mining.