This chapter describes virtual servers, their use and importance in cloud computing, and the technologies that can be used to achieve virtualization, including emulation, para-virtualization, and full virtualization. The chapter focuses on Virtual Machine (VM) technology used in data centers to support full virtualization. It shows how a hypervisor can run multiple operating systems on a single physical server, and explains how applications running on a virtual machine can execute at hardware speed. The chapter goes on to explain how a VM can be stopped, converted to a digital object, sent across a network to another server, and restarted from the point at which it was stopped. The chapter also covers virtual devices, a topic that provides the basis for virtual network and virtual storage devices described in later chapters. Finally, the chapter consider hosted hypervisors that run on top of a conventional operating system.