1. A computer-implemented method for allocating one or more instances of one or more resources to an IT-service, each resource being of one of one or more resource types, each resource instance being provided by one of one or more resource managers of an IT-service management system, the method comprising: analyzing, by a management engine, a service model, the service model specifying the structure of an IT-service and comprising nodes respectively representing one of the resources and respectively being indicative of a resource type of said represented resource, the service model further comprising, for at least one of the nodes, one or more resource management rules specifying the management of said node's resource; and for each of the nodes, executing, by the management engine, at runtime of the IT-service management system: determining the resource type indicated by said node; determining one or more resource management rules assigned to said node; evaluating the resource management rules assigned to said node on a resource instance catalog and the determined resource type, wherein the resource instance catalog is indicative of currently existing instances of the one or more resources and their respective allocation to said IT-service for computing selection criteria; applying the selection criteria on a service provider catalog for selecting one of the one or more resource managers, the service provider catalog being indicative of one or more of the resource managers respectively being operable to provide a resource instance of a given resource type to the IT-service; creating an instance of the resource provided by the selected resource manager; and allocating said instance to the IT-service.