Xen in a virtual machine monitor (VMM) that we have developed at the University of Cambridge over the past 5 years. As a VMM, Xen allows a single computer to be partitioned into a set of isolated virtual computers, each running their own operating system and applications. Xen has received a fair bit of a attention recently and we have even spun out a company to support the commercial use of the software. This article isn't about our VMM, though. Xen is the core part of a much larger vision for public computing that has been behind a lot of our research in the 21st century. In this article, we articulate this vision, and the motivation behind Xen, and cover the details of the current VMM, the context in which it was conceived, and the future uses that we anticipate.