Abstract-There is a group of communication services that use resources from multiple domains in order to deliver their service. Authorization of the end-user is important for such services, because several domains are involved. There are no current solutions for delivering authentication, authorization and accounting (AAA) to multi-domain services. In our study we present two architectures for the delivery of AAA to such services. The architectures are analyzed on their qualitative aspects. A result of this analysis is that direct interconnection of AAA servers is an effective architectural solution. In current multi-domain IP Multimedia Subsystem (IMS) architectures, direct interconnection of AAA servers, such as the Home Subscriber Servers (HSS), is not yet possible. In this paper we argue and recommend to extend the IMS specification by adding a new interface to HSS in order to support the direct interconnection of HSS/AAA servers located in different IMS administrative domains.