• The emergence of new, high-throughput data-collection techniques increasingly allows us to simultaneously interrogate the status of a cell’s components and to determine how and when these molecules interact with each other.
• Various types of molecular interaction webs (including protein–protein interaction, metabolic, signaling and transcription-regulatory networks) emerge from the sum of these interactions that together are principal determinants of the system-scale behaviour of the cell.
• A major challenge of contemporary biology is to embark on an integrated theoretical and experimental programme to map out, understand and model in quantifiable terms the topological and dynamical properties of the various networks that control the behaviour of the cell.
• Here, we review the present knowledge of the design principles for the structure and system-scale function of cellular networks, and the evolutionary mechanisms that might have shaped their development.
• A key insight is that the architectural features of molecular interaction networks within a cell are shared to a large degree by other complex systems, such as the Internet, computer chips or society. This unexpected universality suggests that similar laws govern the development and function of most complex networks in nature.
• Providing that sufficient formalism will be developed this new conceptual framework could potentially revolutionize our view and practice of molecular cell biology.

A key aim of postgenomic biomedical research is to systematically catalogue all molecules and their interactions within a living cell. There is a clear need to understand how these molecules and the interactions between them determine the function of this enormously complex machinery, both in isolation and when surrounded by other cells. Rapid advances in network biology indicate that cellular networks are governed by universal laws and offer a new conceptual framework that could potentially revolutionize our view of biology and disease pathologies in the twenty-first century.