This book explains the principles, foundations and methodologies adopted in data centers to achieve demand-side energy and peak power management. It gives a brief introduction about Smart Grid, how the transition from legacy to Smart Grid is realized, the different approaches for demand-side management (DSM), and then discusses the opportunities of data centers to achieve DSM and highlight the different considered optimization criterion. Data centers are the backbones in realizing digitization where they host ICT (information and communication technologies) resources like servers, storage devices and networking equipment. Despite their advantages in terms of providing numerous services to our modern society (e.g. social media, e-commerce, online learning), the major drawback is that data centers devour enormous amounts of energy. It is expected that the energy usage of data centers will increase in the next few years - expected to reach almost 25% of the worlds overall consumption - due to the emerging and expanding technologies such as Blockchain and 5G.