Manyof the structures that are studied in the social sciences, and in fact in allthe sciences that deal with processes of communication and interaction, takethe form of networks. Formally speaking, a network is a graph (see graphtheory), but the term network is often applied to graphs consisting of many,concrete components connected by many interactions (e.g., networks of individualsentertaining communicative relationships, networks of servers on the Internet). Network theory, which is thus an application of graph theory, focuses on typical properties of complexnetworks, such as the existence of particular nodes that are characterized bymuch more connections with other nodes than average nodes (so-called hubs,which occur in technological networks such as the Internet, but also in socialgroups and the brain).