Neurons are not randomly arranged in the human brain. In the cortex, they are organized in interconnected clusters with high intrinsic connectivity. This modular connectivity structure, in which clusters eventually serve as functional units, is formed in early phases of development. The underlying self-organization process is regulated by neuronal activity but the detailed mechanisms are still poorly understood. Based on in vitro studies and computational modeling, neuroscientists have now made an important contribution to the understanding of brain networks and their development: in their current study, they show how neuronal outgrowth and migration interact in shaping network architecture and the degree of modularity in mature networks.