Current trends in communications technology favour modular, reusable servicecomponents delivered over converged networks. This service oriented approach givesrise to a new breed of composite, personalised services, but so far web-based providersdominate this market. In order to adapt and compete, telecom operators need to implementeffective methods for developing, integrating and delivering services.IP Multimedia Subsystem (IMS) is a fairly new framework being tested and rolledoutby telecom operators around the globe. It has a layered architecture that accommodatesthe delivery of personalised, composite, multimedia services over convergedheterogeneous networks. However, the static nature of IMS service chaining limits itsflexibility.This thesis proposes a model for dynamic service orchestration in IMS centred architecture.The proposal addresses orchestration of typical IMS services running onnative SIP application servers as well as the incorporation of a variety of services residingin foreign domains. In particular, the possibility to include external Web Servicesin composite services the IMS domain is examined. A new hierarchical configurationof service brokers is introduced and a basis prototype is implemented in the scenarioof a composite Presence service.The proposed model in this thesis augments the current IMS service provisioningmechanism in several ways: It introduces the notion of dynamic service brokering.It adds explicit support for non-telecom services in native IMS application servers.Furthermore, the proposed model utilises only reference points already present in theexisting IMS specification, no additional protocols or control functions were needed.The functionality introduced is meant to improve the flexibility of IMS service provisioningin terms of both the type of services that can be offered natively as well as thetypes of services that can be supported from third parties.The intention of this thesis is to provide a model for IMS service orchestration.In particular, it identifies the technologies that make such an architecture feasible aswell as points out best practices for maintaining performance levels. The model wasverified by implementing a prototype that blends native IMS services and externalWebServices in the IMS domain.