This chapter introduces the topic of cloud computing. After giving examples of how cloud can be used, the chapter lists characteristics of cloud facilities, including the ability to scale computing dynamically and only charge a user for the resources used. It summarizes major changes in computing infrastructure over the years and the shift from mainframes to personal computers and smart phones. It describes how the power wall finally forced the computing industry to move to the use of parallelism with multicore processors and multiple computers. The chapter discusses how collecting server computers into a centralized facility can reduce both the cost of equipment and the recurring cost of IT staff. The chapter describes early data centers and load balancers used to provide large-scale web sites. It introduces the idea of server racks that form the basis for cloud data centers. The chapter concludes by noting that cloud moves back to centralization, reversing the decades-long trend toward decentralization.