Distinct cellular functions are executed by separate groups of proteins, organized into complexes or functional modules, which are ultimately interconnected in cell-wide protein networks. Understanding the structures and operational modes of these networks is one of the next great challenges in biology, and microorganisms are at the forefront of research in this field. In this Review, we present our current understanding of bacterial protein networks, their general properties and the tools that are used for systematically mapping and characterizing them. We then discuss two well-studied examples, the chemotaxis network and the cell cycle network in Escherichia coli, to illustrate how network architecture promotes function.