This paper presents state-of-the-art research efforts in implementing a Policy Based architecture for Quality of Service management and provisioning of network services in the IP Multimedia Subsystem (IMS). While Policy Based Network Management has been adopted by the Third Generation Partnership Project and the Telecoms & Internet converged Services & Protocols for Advanced Networks as the solution for managing QoS in the IMS, the proposed mechanisms are sorely lacking in many aspects. First, the policy logic and decision mechanism are not specified, nor is the actual resource allocation procedure. Second, while the IMS is being tailored as the Service Delivery Platform for Next Generation Networks many of the QoS provisioning solutions are designed specifically for UMTS as an access network and need to be enhanced to support other technologies. In the current specifications, 3GPP is already evolving to support Packet Cable, WLAN and WiMAX accesses, but their respective policy-related sections are still void. Furthermore the proposed solutions do not specify how the network processes user requests for services in the IMS infrastructure. This article analyses critical QoS management issues within the IMS, and proposes solutions by designing a Policy Decision Function, which is the central element in the policy architecture. These solutions are presented in a proof of concept implementation running in a real IMS testbed making use of the Open Source IMS Core and considering the management of QoS in different scenarios.