The Peer-to-Peer (P2P) paradigm allows potentially millions of hosts under different administrative controls to join a virtual system where they can exchange resources in a scalable manner and run distributed applications. As a result, wealth of services can be accumulated and disseminated efficiently promoting a huge economic and social impact. Despite the explosive growth in P2P applications, the P2P scalable and flexible architecture has not exhausted its potential. The key objective of this project is to address the challenges and opportunities that face the deployment of structured P2P systems. In particular, the primary focus is on the design and implementation of network protocols supporting new services that complement the traditional, mostly file-sharing applications, new schemes to locate services, new strategies to serve them that compensate for the discrepancies in capacities and intentions of different peers in the system, and a variety of optimizations in order to improve the users' experience. Also, this project introduces the so-called metric-induced topology inference and integration schemes that are used to characterize the impact of Internet traffic on P2P performance as perceived by service consumers, to optimize content distribution, and to support network-aware applications. This project will also deliver a prototype that will be made publicly available and will be deployed as a service to help disseminate the investigated ideas and to lay the ground for testing new ideas. The prototype will also serve a significant educational role as an invaluable tool in the classroom to bring hands-on experience to students about P2P technology.