A controller receives a request for the provision of a slice of a network to a virtual network operator. The controller determines whether the slice can be provided from existing resources, whether the slice can be provided if the existing resources are re-configured, or whether the slice can only be provided if additional resources are obtained. The controller then automatically takes the action required in order to provide the requested slice. The controller can also be used to release resources that are no longer required. Additionally, the controller can be used to re-optimize the network on request.