Abstract. The success of Internet telephony services like Skype illustrates the feasibility of utilizing unstructured Peer-to-Peer (P2P) networks as an economical platform for supporting group communication applications. However, the ad-hoc nature of these networks poses significant challenges to the efficiency and scalability of the group communication services. This paper presents the design and implementation of GroupCast − a utility-aware middleware architecture for scalable and efficient P2P group communications. The GroupCast design is characterized by three unique features. First, we present the utility function for quantifying the role of unicast links in enhancing the scalability and efficiency of the group communication applications. This utility function provides a careful combination of the two most important performance factors, namely relative network locations and resource capabilities of the end hosts. Second, we develop a utility-aware distributed spanning tree const...