In physical systems built with many components, emergent properties, such as magnetism, are often generated from the interactions among these particles. These emergent properties are often invisible when observing individual particles, since they depend on large-scale interactions between them. Likewise, the function of neural circuits has been mostly studied by examining the responses of individual neuron, yet it is probably an emergent property that arises from the coordinated activity of large numbers of neurons. To capture this emergent level of brain function, we proposed to launch a large-scale, international public project, the Brain Activity Map Project (which has become the BRAIN Initiative), aimed at developing new methods to measure and control neural activity across complete neural circuits in experimental animals and human patients. This technological effort will be an interdisciplinary project, incorporating into neuroscience many methods and approaches from the physical sciences and nanotechnologies. The data obtained with these new methodscould prove to be an invaluable step towards understanding fundamental and pathological brain processes.