A Resource Manager is provided for managing a plurality of computers. Each of the computers is operable in one of a plurality of power configurations, including a high-power configuration and low-power configurations. The Manager exchanges messages with a Manager Proxy in each computer. Responsive to notifications from Manager Proxies of changes in execution state of an application, the Manager determines a power configuration applicable for the computer of that Manager Proxy, or for a set of computers executing the application. The Manager indicates the power configuration to the Manager Proxy which then decides whether to implement a change.