Traditionally, networks of complex topology have been described using the random graph theory of Erd ˝os and Renyi (ER) [8,9]. However, while it has been much investigated in combinatorial graph theory, in the absence of data on large networks the predictions of the ER theory were rarely tested in the real world. This is changing very fast lately: driven by the computerization of data acquisition, topological information on various real-world networks is increasingly available. Due to the importance of understanding the topology of some of these systems, it is likely that in the near future
we will witness important advances in this direction. Furthermore, it is also possible that seemingly random networks in Nature have rather complex internal structure, that cover generic features, common to many systems. Uncovering the universal properties characterizing the formation and the topology of complex networks could bring about the much coveted revolution beyond reductionism [1,2].