The IMS offers telecom operators and service providers with the architecture and the procedures to build an open IP-based service infrastructure for deployment of innovative multimedia communication services that integrate telecom and data services. IMS services should be network agnostic and user-centric. This implies a design of IMS services with reusable service enablers. This service design should rely on a common IMS service modeling to ensure interoperability and coherence in the service portfolio of service providers. The article proposes a definition of IMS services and an IMS service modeling that takes into account the user perception of the service and the technical functions carrying out the service.