Over the past decades it has become clear that the metaphor of networks - ensembles of discrete nodes connected by links - offers a powerful conceptual framework for the description and analysis of many real world systems [1-5]. The science of networks has grown into a field which is by now firmly established in several disciplines, including mathematics, physics, biology, computer science, economics and the social sciences. Dramatic progress has been made both in the characterization of real world networks and in the study of dynamical models of networks.