In the previous chapters, we discussed Smart Grid and demand-side management (DSM). This chapter focuses on exploring the opportunities and flexibilities of DSM within the context of data centers. We start by providing a brief overview of the current power grid in Sect. 4.1. Then, in Sect. 4.2, we describe the ICT infrastructure of data centers, including cooling technologies, power sources, and suppliers. Section 4.3 demonstrates the reasons for energy management in data centers and highlights the chances for improvement. Section 4.4 introduces the concept of flexibility and various mechanisms such as workload migration, shifting, consolidation, frequency scaling, management of the cooling system as well as uninterruptible power supply. The chapter is concluded with a summary in Sect. 4.5.