Operating system virtualization has been available on commodity hardware for a few years, and today attracts considerable commercial and research interest. Virtualization allows one or more virtual machines (VMs) to run on a single physical machine, and to interact via virtual devices, such as virtual hard discs or virtual network cards. To model basic virtualization operations, we propose a process calculus, V, with primitives to start and stop VMs, and to read and write data in a hierarchical store. Formalisms such as V may be useful for programming and reasoning about various applications of virtualization, such as VM-based trusted computing or VM-based computational grids.