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Network is a tool to describe systems composed of many different units, which each typically interact with a few of the other units. Networks are thus used to quantify complex systems from the intricate interactions of proteins inside a living cell, to ecosystems, social systems and computer networks. In most cases the network quantifies communication channels in the system. Thus, directly connected nodes communicate easy, while more distant nodes only obtain exchange information through a number of intermediate steps. Many complex systems are associated with information transfer. This includes neural networks with their synaptic rewiring, molecular networks evolved to modulate the protein activity in living cells, and social networks exemplified by the Internet. In Figs. 1 and 2 we show, respectively, the worldwide Internet, a social network, and some examples of molecular networks inside living cells. In all these cases the purpose of the network is to serve as a scaffold for information transfer.