Communication networks are part of our societies' vital infrastructures, and the services they provide are fundamental to our daily work and private lives. The networks are constantly evolving, which cause high uncertainty about current and future network usage and traffic demands. An important trend for the network evolution is that data and wireless networks are increasingly inter-connected with sensor networks, industrial communication systems, as well as ad hoc and femto-cell wireless networks into one big internet. Another important trend is the proliferation of novel application-layer communication and interaction models, including social networks, blogs and micro-blogs, and user-generated content sharing. Many of these models are proliferating on the wired internet and they are increasingly becoming available also on mobile terminals. The traffic patterns resulting from the new interaction models and their dependence on human behavior and social contexts are not well understood, and little is known about the usage of services and the resulting traffic in heterogeneous interconnected network. Given the importance of communication systems in our society, and the tremendous technical changes that constantly occur, this ITC Specialist Seminar has been aimed at the characterization of network usage and traffic, which is germane to the design, operation and management of all forms of communication networks. The seminar takes place on October 8 to 9, 2008, in Berlin, Germany. The program consists of talks for seven papers selected from the open call, five invited talks, two keynote talks and a panel. We are honored to have Prof. Patrick Thiran from EPFL, Switzerland, and Dr. Pablo Rodriguez from Telefonica Research, Spain, as keynote speakers. Patrick's talk has the title "Locating IP congested links with unicast probes," and Pablo's has the title "Delay Tolerant Bulk Data Transfers on the Internet or how to book some terabytes on red-eye' bandwidth"; the topic of the panel discussion is along the theme of the seminar, with the title "Network Usage and Traffic: Do we need fluid mechanics or plumbing?"