Abstract
An ambitious goal of contemporary biological research is the elucidation of the structure and functions of networks that constitute cells and organisms. In biological systems, networks appear in many different disguises, ranging from protein interactions to metabolic networks. The emergence of these networks is driven by self-organizing processes that are governed by simple but generic laws. While unraveling the complex and interwoven systems of different interacting units, it has become clear that the topology of networks of different biological origin share the same characteristics on the large scale. In this chapter, we survey the most prominent characteristics of biological networks, focusing on the emergence of the scale-free architecture and the hierarchical arrangement of modules.