Data sets that illustrate the methods developed in the book are introduced in this chapter. These are the Florentine marriage networks, Zachary’s karate club network, protein–protein interaction data, collaboration networks, gene co-expression networks, Sampson’s monastery networks, Krackhardt’s manager friendship networks, trade networks, Adamic and Glance’s political blog network and the World Wide Web. Networks encountered in practice are usually derived from larger, more comprehensive collections of data and ways of constructing a network from such data are also discussed.