In the Netherlands, Switzerland, UK, and Germany, national networks exist of collaborating excellent research groups in the areas of multimedia content analysis (MCA) and social and peer-to-peer (SP2P) networks. The Network of Excellence PetaMedia brings these four strong national networks together, at first to form a European network of national networks, and eventually to establish a sustainable European virtual centre of excellence to which research groups throughout Europe can connect. The four core partners of the project represent and coordinate the respective national networks; they are responsible for linking up EU national partners to the NoE.The purpose of joining four national networks is to achieve larger momentum, to integrate available resources, and to further develop complementary expertise necessary for pushing new paradigms in enabling efficient and effective access to multimedia content in emerging network structures. The collective research effort that thus comes available will be directed towards integration of existing MCA and SP2P technologies, and towards identification and exploration of potentials and limitations of MCA/SP2P combinations. A particular scientific challenge that binds the partners is the synergetic combination of user-based collaborative tagging, peer-to-peer networks and multimedia content analysis. Solutions and collaborative research field trials will be built on the coordinating partner's open source P2P software Tribler.The NoE will foster the linking up of researchers in the area of MCA and SP2P, resulting in a closer and harmonized collaboration at both European and national level. It is the ambition of the NoE to become an internationally renowned research centre with impact on national and European research funds in the MCA/SP2P area. The centre also takes up the challenge to create international research proposals based on complementary expertise of partners in national research networks (www.petamedia.org).